diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidations.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidations.java
index c80f0c52494d..52069d9bbc97 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidations.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidations.java
@@ -120,7 +120,14 @@ private static ValidationRule.Result checkNullableAttribute(SchemaWrapper schema
         if (schemaWrapper.getOpenAPI() != null) {
             SemVer version = new SemVer(schemaWrapper.getOpenAPI().getOpenapi());
             if (version.atLeast("3.1")) {
-                if (ModelUtils.isNullable(schema)) {
+                // ModelUtils.isNullable checks schema.getNullable(), but swagger-parser does not populate
+                // that field when parsing OAS 3.1 documents — 'nullable' is not a valid 3.1 keyword, so
+                // the parser stores it as a raw extension under the key "nullable" instead.
+                // We must check both paths to catch the deprecated usage in either case,
+                // regardless of whether the value is true or false.
+                boolean hasNullableExtension = schema.getExtensions() != null
+                        && schema.getExtensions().containsKey("nullable");
+                if (ModelUtils.isNullable(schema) || schema.getNullable() != null || hasNullableExtension) {
                     result = new ValidationRule.Fail();
                     result.setDetails(String.format(Locale.ROOT,
                             "OAS document is version '%s'. Schema '%s' uses 'nullable' attribute, which has been deprecated in OAS 3.1.",
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidationsTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidationsTest.java
index ebff8ea7e4b8..a48d714d88a5 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidationsTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidationsTest.java
@@ -1,6 +1,8 @@
 package org.openapitools.codegen.validations.oas;
 
+import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.media.*;
+import org.openapitools.codegen.TestUtils;
 import org.openapitools.codegen.validation.Invalid;
 import org.openapitools.codegen.validation.ValidationResult;
 import org.testng.Assert;
@@ -67,6 +69,51 @@ public void testOneOfWithSiblingPropertiesDisabledRule(Schema schema, boolean ma
         Assert.assertEquals(warnings.size(), 0, "Expected rule to be disabled.");
     }
 
+    /**
+     * The validation warning for 'nullable: true' in an OAS 3.1 spec must fire.
+     * The existing checkNullableAttribute only checked ModelUtils.isNullable(schema) which relies on
+     * schema.getNullable() — but swagger-parser does not populate that field for 3.1 specs (it stores
+     * the value in extensions["nullable"] instead). The fix must also check extensions["nullable"].
+     */
+    @Test(description = "nullable: true in OAS 3.1 spec must trigger the nullable-deprecated warning")
+    public void testNullableAttributeInOas31_triggerWarning() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/nullable-deprecated-in-oas31.yaml");
+        Schema<?> proxyUrl = (Schema<?>) openAPI.getComponents().getSchemas().get("TestModel").getProperties().get("proxyUrl");
+
+        RuleConfiguration config = new RuleConfiguration();
+        config.setEnableRecommendations(true);
+        OpenApiSchemaValidations validator = new OpenApiSchemaValidations(config);
+
+        ValidationResult result = validator.validate(new SchemaWrapper(openAPI, proxyUrl));
+        List<Invalid> nullableWarnings = result.getWarnings().stream()
+                .filter(invalid -> "Schema uses the 'nullable' attribute.".equals(invalid.getRule().getDescription()))
+                .collect(Collectors.toList());
+
+        Assert.assertEquals(nullableWarnings.size(), 1,
+                "Expected exactly one 'nullable attribute deprecated' warning for a 3.1 spec using nullable: true");
+    }
+
+    /**
+     * The nullable-deprecated warning must NOT fire for an OAS 3.1 spec using the correct 3.1 null type syntax.
+     */
+    @Test(description = "correct OAS 3.1 null type syntax (type: [string, null]) must NOT trigger the nullable-deprecated warning")
+    public void testNullTypeInOas31_noWarning() {
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/null-types-simple.yaml");
+        Schema<?> stringDataOrNull = (Schema<?>) openAPI.getComponents().getSchemas().get("WithNullableType").getProperties().get("stringDataOrNull");
+
+        RuleConfiguration config = new RuleConfiguration();
+        config.setEnableRecommendations(true);
+        OpenApiSchemaValidations validator = new OpenApiSchemaValidations(config);
+
+        ValidationResult result = validator.validate(new SchemaWrapper(openAPI, stringDataOrNull));
+        List<Invalid> nullableWarnings = result.getWarnings().stream()
+                .filter(invalid -> "Schema uses the 'nullable' attribute.".equals(invalid.getRule().getDescription()))
+                .collect(Collectors.toList());
+
+        Assert.assertEquals(nullableWarnings.size(), 0,
+                "OAS 3.1 with type:[string,null] must not trigger the nullable-deprecated warning");
+    }
+
     @DataProvider(name = "apacheNginxRecommendationExpectations")
     public Object[][] apacheNginxRecommendationExpectations() {
         return new Object[][]{
diff --git a/modules/openapi-generator/src/test/resources/3_1/nullable-deprecated-in-oas31.yaml b/modules/openapi-generator/src/test/resources/3_1/nullable-deprecated-in-oas31.yaml
new file mode 100644
index 000000000000..5e3c2ddbec92
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_1/nullable-deprecated-in-oas31.yaml
@@ -0,0 +1,14 @@
+openapi: 3.1.0
+info:
+  title: Nullable deprecated in OAS 3.1 test
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    TestModel:
+      type: object
+      properties:
+        # OAS 3.0 'nullable: true' used in a 3.1 spec — deprecated and should trigger a warning
+        proxyUrl:
+          type: string
+          nullable: true