diff --git a/src/core/jsonschema/known_resolver.cmake b/src/core/jsonschema/known_resolver.cmake
index 991dda43f..4b1197b6c 100644
--- a/src/core/jsonschema/known_resolver.cmake
+++ b/src/core/jsonschema/known_resolver.cmake
@@ -91,6 +91,11 @@ set(OPENAPI_3_1_DIR "${PROJECT_SOURCE_DIR}/vendor/openapi/oas/3.1")
 file(READ "${OPENAPI_3_1_DIR}/meta/base" OPENAPI_OAS_3_1_META_BASE)
 file(READ "${OPENAPI_3_1_DIR}/dialect/base" OPENAPI_OAS_3_1_DIALECT_BASE)
 
+# OpenAPI v3.0
+set(OPENAPI_3_0_DIR "${PROJECT_SOURCE_DIR}/vendor/openapi/oas/3.0")
+file(READ "${OPENAPI_3_0_DIR}/schema/2024-10-18" OPENAPI_OAS_3_0_SCHEMA_2024_10_18)
+file(READ "${OPENAPI_3_0_DIR}/schema/2021-09-28" OPENAPI_OAS_3_0_SCHEMA_2021_09_28)
+
 if(NOT KNOWN_RESOLVER_INPUT)
   message(FATAL_ERROR "Missing KNOWN_RESOLVER_INPUT")
 endif()
diff --git a/src/core/jsonschema/known_resolver.in.cc b/src/core/jsonschema/known_resolver.in.cc
index e4af4a56e..fa2096c64 100644
--- a/src/core/jsonschema/known_resolver.in.cc
+++ b/src/core/jsonschema/known_resolver.in.cc
@@ -69,6 +69,9 @@ enum class KnownSchema : std::uint8_t {
   OAS_3_1_DIALECT_BASE,
   OAS_3_1_META_BASE,
 
+  OAS_3_0_DIALECT_2024_10_18,
+  OAS_3_0_DIALECT_2021_09_28,
+
   UNKNOWN
 };
 
@@ -419,6 +422,19 @@ static auto parse_identifier(const std::string_view identifier) -> KnownSchema {
     return KnownSchema::OAS_3_1_META_BASE;
   }
 
+  // OpenAPI v3.0
+  //
+  // The OpenAPI Initiative never assigned a dialect identifier to the v3.0
+  // Schema Object, unlike v3.1 and later, so we mint one per published release
+  // of the document schema that embeds it, as described in RFC 4151
+  if (identifier == "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect") {
+    return KnownSchema::OAS_3_0_DIALECT_2024_10_18;
+  }
+
+  if (identifier == "tag:spec.openapis.org,2021-09-28:oas/3.0/dialect") {
+    return KnownSchema::OAS_3_0_DIALECT_2021_09_28;
+  }
+
   return KnownSchema::UNKNOWN;
 }
 
@@ -700,6 +716,34 @@ auto sourcemeta::core::schema_resolver(const std::string_view identifier)
           sourcemeta::core::parse_json(R"EOF(@OPENAPI_OAS_3_1_META_BASE@)EOF")};
       return SCHEMA;
     }
+    // The document schema is embedded rather than registered on its own, so
+    // that the dialect identifier is the only thing this resolver exposes. Its
+    // inner identifier must be kept for the references it makes to itself to
+    // keep resolving against its own scope
+    case KnownSchema::OAS_3_0_DIALECT_2024_10_18: {
+      static const auto SCHEMA{sourcemeta::core::parse_json(R"EOF({
+  "id": "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect",
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "description": "The OpenAPI v3.0.x Schema Object dialect",
+  "allOf": [ { "$ref": "#/definitions/openapi/definitions/Schema" } ],
+  "definitions": {
+    "openapi": @OPENAPI_OAS_3_0_SCHEMA_2024_10_18@
+  }
+})EOF")};
+      return SCHEMA;
+    }
+    case KnownSchema::OAS_3_0_DIALECT_2021_09_28: {
+      static const auto SCHEMA{sourcemeta::core::parse_json(R"EOF({
+  "id": "tag:spec.openapis.org,2021-09-28:oas/3.0/dialect",
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "description": "The OpenAPI v3.0.x Schema Object dialect",
+  "allOf": [ { "$ref": "#/definitions/openapi/definitions/Schema" } ],
+  "definitions": {
+    "openapi": @OPENAPI_OAS_3_0_SCHEMA_2021_09_28@
+  }
+})EOF")};
+      return SCHEMA;
+    }
     case KnownSchema::UNKNOWN:
       return std::nullopt;
   }
@@ -771,6 +815,8 @@ auto sourcemeta::core::schema_is_official(
     case KnownSchema::OAS_3_2_META_2025_09_17:
     case KnownSchema::OAS_3_1_DIALECT_BASE:
     case KnownSchema::OAS_3_1_META_BASE:
+    case KnownSchema::OAS_3_0_DIALECT_2024_10_18:
+    case KnownSchema::OAS_3_0_DIALECT_2021_09_28:
     case KnownSchema::UNKNOWN:
       return false;
   }
diff --git a/test/jsonschema/jsonschema_resolver_test.cc b/test/jsonschema/jsonschema_resolver_test.cc
index 1cab48a8c..1bfc3af57 100644
--- a/test/jsonschema/jsonschema_resolver_test.cc
+++ b/test/jsonschema/jsonschema_resolver_test.cc
@@ -423,6 +423,31 @@ TEST(openapi_3_2) {
   EXPECT_SCHEMA("https://spec.openapis.org/oas/3.2/meta/2025-09-17");
 }
 
+TEST(openapi_3_0) {
+  EXPECT_SCHEMA("tag:spec.openapis.org,2024-10-18:oas/3.0/dialect");
+  EXPECT_SCHEMA("tag:spec.openapis.org,2021-09-28:oas/3.0/dialect");
+}
+
+TEST(openapi_3_0_document_schema_is_encapsulated) {
+  EXPECT_FALSE(sourcemeta::core::schema_resolver(
+                   "https://spec.openapis.org/oas/3.0/schema/2024-10-18")
+                   .has_value());
+  EXPECT_FALSE(sourcemeta::core::schema_resolver(
+                   "https://spec.openapis.org/oas/3.0/schema/2021-09-28")
+                   .has_value());
+}
+
+TEST(openapi_3_0_dialect_points_to_the_schema_object) {
+  const auto result{sourcemeta::core::schema_resolver(
+      "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect")};
+  EXPECT_TRUE(result.has_value());
+  const sourcemeta::core::JSON &document{result.value()};
+  EXPECT_EQ(document.at("allOf").at(0).at("$ref").to_string(),
+            "#/definitions/openapi/definitions/Schema");
+  EXPECT_EQ(document.at("definitions").at("openapi").at("id").to_string(),
+            "https://spec.openapis.org/oas/3.0/schema/2024-10-18");
+}
+
 TEST(idempotency) {
   EXPECT_SCHEMA("https://json-schema.org/draft/2020-12/schema");
   EXPECT_SCHEMA("https://json-schema.org/draft/2020-12/schema");
@@ -582,6 +607,21 @@ TEST(is_known_schema_openapi_3_2) {
                   .has_value());
 }
 
+TEST(is_known_schema_openapi_3_0) {
+  EXPECT_TRUE(sourcemeta::core::schema_is_known(
+      "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect"));
+  EXPECT_TRUE(sourcemeta::core::schema_resolver(
+                  "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect")
+                  .has_value());
+  EXPECT_TRUE(sourcemeta::core::schema_is_known(
+      "tag:spec.openapis.org,2021-09-28:oas/3.0/dialect"));
+  EXPECT_TRUE(sourcemeta::core::schema_resolver(
+                  "tag:spec.openapis.org,2021-09-28:oas/3.0/dialect")
+                  .has_value());
+  EXPECT_FALSE(sourcemeta::core::schema_is_known(
+      "https://spec.openapis.org/oas/3.0/schema/2024-10-18"));
+}
+
 TEST(is_known_schema_unknown) {
   EXPECT_FALSE(sourcemeta::core::schema_is_known("https://example.com/foobar"));
   EXPECT_FALSE(sourcemeta::core::schema_resolver("https://example.com/foobar")
@@ -746,6 +786,13 @@ TEST(is_official_schema_openapi_3_2) {
       "https://spec.openapis.org/oas/3.2/meta/2025-09-17"));
 }
 
+TEST(is_official_schema_openapi_3_0) {
+  EXPECT_FALSE(sourcemeta::core::schema_is_official(
+      "tag:spec.openapis.org,2024-10-18:oas/3.0/dialect"));
+  EXPECT_FALSE(sourcemeta::core::schema_is_official(
+      "tag:spec.openapis.org,2021-09-28:oas/3.0/dialect"));
+}
+
 TEST(is_official_schema_unknown) {
   EXPECT_FALSE(
       sourcemeta::core::schema_is_official("https://example.com/foobar"));