diff --git a/ext/json/ext/parser/parser.c b/ext/json/ext/parser/parser.c
index a740bd42..fc16f727 100644
--- a/ext/json/ext/parser/parser.c
+++ b/ext/json/ext/parser/parser.c
@@ -1457,18 +1457,21 @@ static void json_ensure_eof(JSON_ParserState *state)
 
 static VALUE convert_encoding(VALUE source)
 {
-  int encindex = RB_ENCODING_GET(source);
+    StringValue(source);
+    int encindex = RB_ENCODING_GET(source);
 
-  if (RB_LIKELY(encindex == utf8_encindex)) {
-    return source;
-  }
+    if (RB_LIKELY(encindex == utf8_encindex)) {
+        return source;
+    }
 
- if (encindex == binary_encindex) {
-    // For historical reason, we silently reinterpret binary strings as UTF-8
-    return rb_enc_associate_index(rb_str_dup(source), utf8_encindex);
-  }
+    if (encindex == binary_encindex) {
+        // For historical reason, we silently reinterpret binary strings as UTF-8
+        return rb_enc_associate_index(rb_str_dup(source), utf8_encindex);
+    }
 
-  return rb_funcall(source, i_encode, 1, Encoding_UTF_8);
+    source = rb_funcall(source, i_encode, 1, Encoding_UTF_8);
+    StringValue(source);
+    return source;
 }
 
 struct parser_config_init_args {
@@ -1583,10 +1586,16 @@ static VALUE cParserConfig_initialize(VALUE self, VALUE opts)
     return self;
 }
 
-static VALUE cParser_parse(JSON_ParserConfig *config, VALUE Vsource)
+static VALUE cParser_parse(JSON_ParserConfig *config, VALUE src)
 {
-    Vsource = convert_encoding(StringValue(Vsource));
-    StringValue(Vsource);
+    VALUE Vsource = convert_encoding(src);
+
+    // Ensure the string isn't mutated under us.
+    // The classic API to use is `rb_str_locktmp`, but then we'd
+    // need to use `rb_protect` to make sure we always unlock.
+    if (Vsource == src) {
+        Vsource = rb_str_new_frozen(Vsource);
+    }
 
     VALUE rvalue_stack_buffer[RVALUE_STACK_INITIAL_CAPA];
     rvalue_stack stack = {
@@ -1597,6 +1606,7 @@ static VALUE cParser_parse(JSON_ParserConfig *config, VALUE Vsource)
 
     long len;
     const char *start;
+
     RSTRING_GETMEM(Vsource, start, len);
 
     VALUE stack_handle = 0;
@@ -1615,6 +1625,7 @@ static VALUE cParser_parse(JSON_ParserConfig *config, VALUE Vsource)
     // it won't cause a leak.
     rvalue_stack_eagerly_release(stack_handle);
     RB_GC_GUARD(stack_handle);
+    RB_GC_GUARD(Vsource);
     json_ensure_eof(state);
 
     return result;
@@ -1635,9 +1646,6 @@ static VALUE cParserConfig_parse(VALUE self, VALUE Vsource)
 
 static VALUE cParser_m_parse(VALUE klass, VALUE Vsource, VALUE opts)
 {
-    Vsource = convert_encoding(StringValue(Vsource));
-    StringValue(Vsource);
-
     JSON_ParserConfig _config = {0};
     JSON_ParserConfig *config = &_config;
     parser_config_init(config, opts, false);
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index 67d86a0b..1969e79b 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -878,6 +878,22 @@ def test_frozen
     end
   end
 
+  def test_mutating_source_string_during_parsing
+    expected = ([1] * 100) + [2.3] + ([1] * 100)
+    source = JSON.generate(expected)
+    expected.delete_at(100)
+
+    fake_decimal_class = Class.new
+    fake_decimal_class.define_method(:initialize) do |number|
+      source.tr!('1', '0')
+      number.to_f
+    end
+
+    actual = JSON.parse(source, decimal_class: fake_decimal_class)
+    actual.delete_at(100)
+    assert_equal expected, actual
+  end
+
   private
 
   def assert_equal_float(expected, actual, delta = 1e-2)