diff --git a/README.md b/README.md
index 245cb6305..55744dcc7 100644
--- a/README.md
+++ b/README.md
@@ -1090,6 +1090,27 @@ end
 
 A parent with several such children sorts by the smallest value ascending and the largest descending. `:score => :min`, `:max`, `:avg` or `:total` combines them another way. Solr compares the values as floats, so times less than about two minutes apart, and integers above 2**24, can tie. Solr also reads a negative value as zero, so times before 1970 all sort as 1970, and negative numbers as zero. A later `order_by` breaks the tie.
 
+#### Faceting on a child field
+
+**Solr 7.7 and above**, for the `uniqueBlock` count: Solr 7.7 has it and 7.3 doesn't. The specs run it on Solr 8 and above.
+
+`child_facet` facets on a field of the children, counting the parents that have each value rather than the children, and ranking the values by that count, so `:limit` keeps the values most parents have. It counts only the children of parents matching the search, and only those meeting the block's restrictions, the same restrictions `with_child` takes. A `with_child` on the search doesn't restrict the facet's children, so repeat any condition they need in the block.
+
+```ruby
+# The names of the milestones Ana owns on active projects, each with its number of projects
+search = Project.search do
+  with :status, 'active'
+  child_facet :milestones, :name, :matches => 'des.*' do
+    with :owner_names, 'ana'
+  end
+end
+search.facet(:name).rows.map { |row| [row.value, row.count] } # => [["design", 1]]
+```
+
+* The facet is read with `facet`, by the field's name or by `:name`. A search with another facet of the same name raises `ArgumentError` unless `child_facet` has a `:name` of its own.
+* Options: `:limit`, `:offset`, `:prefix`, `:missing`, `:all_buckets`, `:sort` (`:count`, the default, which ranks by the number of parents, or `:index`) with `:sort_type`, `:minimum_count`, counted in parents, and `:matches`. Any other option raises `ArgumentError`.
+* `:matches` is a Lucene regular expression, not a Ruby one, matched against the whole value of a string field, case-sensitively. `Sunspot::Util.escape_lucene_regexp` escapes text to match it literally, e.g. text a user typed. On a multiple-value field, `:matches` selects children, so a matching child's other values are counted too.
+
 #### Schema
 
 The schema needs a `_root_` field with the same type as `id`. The bundled configset has one.
diff --git a/sunspot/lib/sunspot/dsl/field_query.rb b/sunspot/lib/sunspot/dsl/field_query.rb
index 52933253e..11287f436 100644
--- a/sunspot/lib/sunspot/dsl/field_query.rb
+++ b/sunspot/lib/sunspot/dsl/field_query.rb
@@ -388,6 +388,41 @@ def json_facet(*field_names)
         end
       end
 
+      #
+      # Facet on a field of the children in a nested association, counting the
+      # documents that have each value rather than the children, and ranking
+      # the values by that count. Only the children of documents matching the
+      # search are counted, and only those that satisfy every restriction in
+      # the block, which takes the same restrictions as DSL::Scope#with_child.
+      #
+      # The facet is read as <tt>search.facet(field_name)</tt>, or by its
+      # +:name+, which it needs when the search has another facet of that
+      # name. Options: +:limit+, +:offset+, +:prefix+, +:missing+,
+      # +:all_buckets+, +:sort+ (+:count+, the default, or +:index+) with
+      # +:sort_type+, +:minimum_count+ (of documents), and +:matches+: a
+      # Lucene regular expression the child's value of a string field must
+      # match as a whole, case-sensitively. On a multiple-value field,
+      # +:matches+ selects children, so a matching child's other values are
+      # counted too. Util.escape_lucene_regexp escapes text to match literally.
+      #
+      # ==== Example
+      #
+      #   search = Sunspot.search(Project) do
+      #     with :status, 'active'
+      #     child_facet :milestones, :name, :matches => 'des.*' do
+      #       with :owner_names, 'ana'
+      #     end
+      #   end
+      #   search.facet(:name).rows # each row's count is a number of projects
+      #
+      def child_facet(name, field_name, options = {}, &block)
+        block_join, child_setup = build_block_join(false, name, &block)
+        field = child_setup.field(field_name.to_sym)
+        facet = Sunspot::Query::ChildFieldJsonFacet.new(field, options, child_setup, block_join)
+        @search.add_child_json_facet(field, options)
+        @query.add_query_facet(facet)
+      end
+
       def stats(*field_names, &block)
         options = Sunspot::Util.extract_options_from(field_names)
 
diff --git a/sunspot/lib/sunspot/query.rb b/sunspot/lib/sunspot/query.rb
index 6cdae57d7..c4d1437c8 100644
--- a/sunspot/lib/sunspot/query.rb
+++ b/sunspot/lib/sunspot/query.rb
@@ -1,5 +1,5 @@
 %w(filter abstract_field_facet abstract_json_field_facet connective boost_query date_field_facet field_json_facet
-   range_facet range_json_facet date_field_json_facet abstract_fulltext dismax join block_join
+   range_facet range_json_facet date_field_json_facet abstract_fulltext dismax join block_join child_field_json_facet
    field_list field_facet highlighting pagination restriction common_query spellcheck
    standard_query more_like_this more_like_this_query geo geofilt bbox query_facet
    scope sort sort_composite text_field_boost function_query field_stats
diff --git a/sunspot/lib/sunspot/query/abstract_json_field_facet.rb b/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
index 34e094973..8c6d8c2e4 100644
--- a/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
+++ b/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
@@ -1,3 +1,5 @@
+require 'json' # to_params serializes the facet with to_json
+
 module Sunspot
   module Query
     class AbstractJsonFieldFacet
diff --git a/sunspot/lib/sunspot/query/block_join.rb b/sunspot/lib/sunspot/query/block_join.rb
index 47f158aac..7910896ae 100644
--- a/sunspot/lib/sunspot/query/block_join.rb
+++ b/sunspot/lib/sunspot/query/block_join.rb
@@ -43,12 +43,14 @@ def negate
         self.class.new(@nested_setup, !negated?, @scope, @paths)
       end
 
-      private
-
+      # The query matching the children this block join is about: those in
+      # its association that satisfy every restriction in #scope.
       def child_query
         @scope.to_boolean_phrase
       end
 
+      private
+
       def escape(value)
         self.class.escape(value)
       end
diff --git a/sunspot/lib/sunspot/query/child_field_json_facet.rb b/sunspot/lib/sunspot/query/child_field_json_facet.rb
new file mode 100644
index 000000000..d19de9093
--- /dev/null
+++ b/sunspot/lib/sunspot/query/child_field_json_facet.rb
@@ -0,0 +1,55 @@
+module Sunspot
+  module Query
+    #
+    # A terms facet over the children of the matching documents: the children
+    # in one nested association that satisfy a block join's restrictions. Each
+    # row counts the documents with such a child, not the children, and the
+    # rows are ranked by that count.
+    #
+    # With +:matches+, only children whose value of the field matches it are
+    # counted. It is a Lucene regular expression, matched against the whole
+    # indexed value, case-sensitively. Util.escape_lucene_regexp escapes text
+    # to match literally.
+    #
+    class ChildFieldJsonFacet < FieldJsonFacet
+      OPTIONS = [:name, :limit, :offset, :sort, :sort_type, :minimum_count, :prefix, :missing, :all_buckets, :matches].freeze
+
+      # The count each row reports: the number of parents with such a child
+      PARENT_COUNT = 'distinct'.freeze
+
+      attr_reader :name
+
+      def initialize(field, options, setup, block_join)
+        unknown = options.keys - OPTIONS
+        raise ArgumentError, "child_facet doesn't take #{unknown.map(&:inspect).join(', ')}" if unknown.any?
+        unless field.type.is_a?(Sunspot::Type::StringType) || !options.key?(:matches)
+          raise ArgumentError, "child_facet's :matches needs a string field, but #{field.name} isn't one"
+        end
+        raise ArgumentError, "child_facet's :matches takes a Lucene regular expression as a String" if options.key?(:matches) && !options[:matches].is_a?(String)
+
+        super(field, options, setup)
+        @block_join = block_join
+        @name = (options[:name] || field.name).to_sym
+      end
+
+      def field_name_with_local_params
+        params = init_params.merge(:type => 'terms', :field => @field.indexed_name)
+        # Solr ranks buckets by its own count, the children, unless told to use the parent count
+        params[:sort] = { PARENT_COUNT => @options[:sort_type] || 'desc' } if @options[:sort].nil? || @options[:sort] == :count
+        params.delete(:mincount) # Solr's mincount counts children; rows apply :minimum_count to parents
+
+        filters = [@block_join.child_query]
+        filters << "#{@field.indexed_name}:/#{@options[:matches].gsub('/', '\\/')}/" if @options[:matches]
+
+        { @name => params.merge(
+          :domain => { :blockChildren => BlockJoin::ROOTS, :filter => filters },
+          :facet => { PARENT_COUNT => 'uniqueBlock(_root_)' }
+        ) }
+      end
+
+      def get_params
+        field_name_with_local_params
+      end
+    end
+  end
+end
diff --git a/sunspot/lib/sunspot/search.rb b/sunspot/lib/sunspot/search.rb
index 945fb7c4f..c07776c19 100644
--- a/sunspot/lib/sunspot/search.rb
+++ b/sunspot/lib/sunspot/search.rb
@@ -1,5 +1,5 @@
 %w(abstract_search standard_search more_like_this_search query_facet
-   field_facet field_json_facet date_facet range_facet json_facet_stats
+   field_facet field_json_facet child_field_json_facet date_facet range_facet json_facet_stats
    facet_row json_facet_row hit highlight field_group group hit_enumerable
    stats_row stats_json_row field_stats stats_facet query_group).each do |file|
   require File.join(File.dirname(__FILE__), 'search', file)
diff --git a/sunspot/lib/sunspot/search/abstract_search.rb b/sunspot/lib/sunspot/search/abstract_search.rb
index ad71d1c3b..b0cdffb8f 100644
--- a/sunspot/lib/sunspot/search/abstract_search.rb
+++ b/sunspot/lib/sunspot/search/abstract_search.rb
@@ -268,6 +268,12 @@ def add_json_facet(field, options = {})
         add_facet(name, FieldJsonFacet.new(field, self, options))
       end
 
+      def add_child_json_facet(field, options = {}) #:nodoc:
+        name = (options[:name] || field.name).to_sym
+        raise ArgumentError, "The search already has a facet named #{name}; give child_facet a :name of its own" if @facets_by_name.key?(name)
+        add_facet(name, ChildFieldJsonFacet.new(field, self, options))
+      end
+
       def highlights_for(doc) #:nodoc:
         if @solr_result['highlighting']
           @solr_result['highlighting'][doc['id']]
diff --git a/sunspot/lib/sunspot/search/child_field_json_facet.rb b/sunspot/lib/sunspot/search/child_field_json_facet.rb
new file mode 100644
index 000000000..94fca276a
--- /dev/null
+++ b/sunspot/lib/sunspot/search/child_field_json_facet.rb
@@ -0,0 +1,28 @@
+module Sunspot
+  module Search
+    #
+    # The rows of a Query::ChildFieldJsonFacet, read by its name. Each row's
+    # count is a number of parents, and the rows come in Solr's order: by that
+    # count, unless the facet was sorted by :index.
+    #
+    class ChildFieldJsonFacet < FieldJsonFacet
+      def initialize(field, search, options)
+        super
+        @name = (options[:name] || field.name).to_sym
+      end
+
+      def rows
+        @rows ||= begin
+          data = no_data? ? [] : @search.json_facet_response[@name.to_s]['buckets']
+          rows = data.map {|bucket| JsonFacetRow.new(bucket, self) }
+          rows = rows.select {|row| row.count >= @options[:minimum_count] } if @options[:minimum_count]
+          rows
+        end
+      end
+
+      def no_data?
+        @search.json_facet_response[@name.to_s].nil?
+      end
+    end
+  end
+end
diff --git a/sunspot/lib/sunspot/util.rb b/sunspot/lib/sunspot/util.rb
index e83be0378..d244f06b8 100644
--- a/sunspot/lib/sunspot/util.rb
+++ b/sunspot/lib/sunspot/util.rb
@@ -188,6 +188,15 @@ def escape(value)
         RSolr.solr_escape(value).gsub(/([\s\.])/, '\\\\\1')
       end
 
+      #
+      # Escapes text for a Lucene regular expression, such as child_facet's
+      # +:matches+, so it matches literally. Leaves a slash, which child_facet
+      # escapes itself.
+      #
+      def escape_lucene_regexp(value)
+        value.to_s.gsub(/[.?+*|{}\[\]()"\\#@&<>~^$]/) {|char| "\\#{char}" }
+      end
+
       def parse_json_facet(field_name, options, setup)
         field = setup.field(field_name)
         if options[:range] || options[:time_range]
diff --git a/sunspot/spec/api/nested_documents_spec.rb b/sunspot/spec/api/nested_documents_spec.rb
index 15f457154..f3d5fd072 100644
--- a/sunspot/spec/api/nested_documents_spec.rb
+++ b/sunspot/spec/api/nested_documents_spec.rb
@@ -334,6 +334,68 @@ def last_search
     end
   end
 
+  describe 'faceting on a child field' do
+    def json_facet
+      JSON.parse(connection.searches.last['json.facet'])
+    end
+
+    it 'facets on the children of the matching parents, ranked by the number of parents' do
+      session.search(Project) { child_facet(:milestones, :name) { with :owner_names, 'ana' } }
+      expect(json_facet).to eq('name' => {
+        'type' => 'terms', 'field' => 'name_s', 'sort' => { 'distinct' => 'desc' },
+        'domain' => { 'blockChildren' => roots, 'filter' => ['(_sunspot_nested_path_s:"Project.milestones" AND owner_names_sm:ana)'] },
+        'facet' => { 'distinct' => 'uniqueBlock(_root_)' }
+      })
+    end
+
+    it 'is keyed by its :name' do
+      session.search(Project) { child_facet(:milestones, :name, :name => :milestone_name) }
+      expect(json_facet.keys).to eq(['milestone_name'])
+    end
+
+    it 'ranks by the number of parents when sorted by count, and keeps an index sort' do
+      session.search(Project) { child_facet(:milestones, :name, :sort => :count, :sort_type => 'asc') }
+      expect(json_facet['name']['sort']).to eq('distinct' => 'asc')
+
+      session.search(Project) { child_facet(:milestones, :name, :sort => :index, :sort_type => 'asc') }
+      expect(json_facet['name']['sort']).to eq('index' => 'asc')
+    end
+
+    it 'leaves a minimum count to the rows, since Solr would apply it to the children' do
+      session.search(Project) { child_facet(:milestones, :name, :minimum_count => 2) }
+      expect(json_facet['name']).not_to have_key('mincount')
+    end
+
+    it 'filters the children by :matches, escaping a slash' do
+      session.search(Project) { child_facet(:milestones, :name, :matches => 'a/b.*') }
+      expect(json_facet['name']['domain']['filter'].last).to eq('name_s:/a\/b.*/')
+    end
+
+    it 'reads the children of every searched class that declares the association' do
+      session.search(Project, Program) { child_facet(:milestones, :name) }
+      expect(json_facet['name']['domain']['filter'].first).to include('Project.milestones', 'Program.milestones')
+    end
+
+    it 'raises for an option it does not support' do
+      expect { session.search(Project) { child_facet(:milestones, :name, :range => [1, 10]) } }.to raise_error(ArgumentError, /:range/)
+    end
+
+    it 'raises for :matches on a field that is not a string, or given anything but a String' do
+      expect { session.search(Project) { child_facet(:milestones, :started_at, :matches => '2026.*') } }.to raise_error(ArgumentError, /string field/)
+      expect { session.search(Project) { child_facet(:milestones, :name, :matches => /des/) } }.to raise_error(ArgumentError, /String/)
+    end
+
+    it 'raises for a name the search already has a facet by' do
+      expect { session.search(Project) { json_facet :name; child_facet(:milestones, :name) } }.to raise_error(ArgumentError, /:name of its own/)
+    end
+  end
+
+  describe 'escaping a Lucene regular expression' do
+    it 'escapes every character the syntax reserves, but a slash' do
+      expect(Sunspot::Util.escape_lucene_regexp('a.b (c) "d" <e> f&g ~h @i #j \\ k/l')).to eq('a\.b \(c\) \"d\" \<e\> f\&g \~h \@i \#j \\\\ k/l')
+    end
+  end
+
   describe 'removal' do
     it 'removes the whole block when removing a parent' do
       project = Project.new
diff --git a/sunspot/spec/integration/nested_documents_spec.rb b/sunspot/spec/integration/nested_documents_spec.rb
index 595217e0c..a6d3b57ed 100644
--- a/sunspot/spec/integration/nested_documents_spec.rb
+++ b/sunspot/spec/integration/nested_documents_spec.rb
@@ -105,6 +105,69 @@ def solr_count(query)
     end
   end
 
+  describe 'child_facet' do
+    before { skip_before_solr 8, 'The parent count needs uniqueBlock, which Solr 7.7 has and 7.3 lacks, and skip_before_solr compares major versions' }
+
+    def facet_rows(&block)
+      Sunspot.search(Project, &block).facet(:name).rows.map { |row| [row.value, row.count] }
+    end
+
+    it 'counts the parents with a child of each value, not the children' do
+      Sunspot.index!(Project.new(:name => 'built twice', :status => 'active', :milestones => [milestone('build', Time.utc(2026, 3, 1)), milestone('build', Time.utc(2026, 5, 1))]))
+
+      expect(facet_rows { child_facet(:milestones, :name) }).to match_array([['design', 2], ['build', 2]])
+    end
+
+    it 'counts only the children meeting every condition in the block' do
+      expect(facet_rows { child_facet(:milestones, :name) { with :owner_names, 'ana' } }).to match_array([['design', 1], ['build', 1]])
+    end
+
+    it 'counts only the children of parents matching the search' do
+      expect(facet_rows { with :name, 'designed earlier'; child_facet(:milestones, :name) }).to match_array([['design', 1], ['build', 1]])
+    end
+
+    it 'counts only the children whose value matches the regular expression' do
+      expect(facet_rows { child_facet(:milestones, :name, :matches => 'des.*') }).to eq([['design', 2]])
+    end
+
+    it 'matches a value containing a slash' do
+      Sunspot.index!(Project.new(:name => 'slashed', :status => 'active', :milestones => [milestone('a/b', Time.utc(2026, 3, 1))]))
+
+      expect(facet_rows { child_facet(:milestones, :name, :matches => 'a/b') }).to eq([['a/b', 1]])
+    end
+
+    it 'ranks the values by their number of parents, so a limit keeps the values most parents have' do
+      Sunspot.index!(Project.new(:name => 'reviewed thrice', :status => 'active', :milestones => [milestone('review', Time.utc(2026, 3, 1)), milestone('review', Time.utc(2026, 4, 1)), milestone('review', Time.utc(2026, 5, 1))]))
+
+      expect(facet_rows { child_facet(:milestones, :name, :limit => 1) }).to eq([['design', 2]])
+    end
+
+    it 'applies a minimum count to the number of parents' do
+      Sunspot.index!(Project.new(:name => 'built thrice', :status => 'active', :milestones => [milestone('build', Time.utc(2026, 3, 1)), milestone('build', Time.utc(2026, 4, 1)), milestone('build', Time.utc(2026, 5, 1))]))
+
+      expect(facet_rows { child_facet(:milestones, :name, :minimum_count => 3) }).to eq([])
+      expect(facet_rows { child_facet(:milestones, :name, :minimum_count => 2) }).to match_array([['design', 2], ['build', 2]])
+    end
+
+    it 'keeps two facets on the same field apart by their :name' do
+      search = Sunspot.search(Project) do
+        child_facet(:milestones, :name, :name => :anas) { with :owner_names, 'ana' }
+        child_facet(:milestones, :name, :name => :bens) { with :owner_names, 'ben' }
+      end
+
+      expect(search.facet(:anas).rows.map { |row| [row.value, row.count] }).to match_array([['design', 1], ['build', 1]])
+      expect(search.facet(:bens).rows.map { |row| [row.value, row.count] }).to eq([['design', 1]])
+    end
+
+    it 'counts the other values of a multiple-value field on a child that matches' do
+      Sunspot.index!(Project.new(:name => 'shared', :status => 'active', :milestones => [milestone('plan', Time.utc(2026, 3, 1), :owner_names => %w(cy dee))]))
+
+      rows = Sunspot.search(Project) { child_facet(:milestones, :owner_names, :matches => 'd.*') }.facet(:owner_names).rows
+
+      expect(rows.map(&:value)).to match_array(['cy', 'dee'])
+    end
+  end
+
   describe 'order_by_child' do
     before { skip_before_solr 8, 'Solr 6 and earlier reject the sort, and Solr 7 orders some parents differently' }