diff --git a/internal/descriptor/registry.go b/internal/descriptor/registry.go
index e6f78482ef1..3a35dea6229 100644
--- a/internal/descriptor/registry.go
+++ b/internal/descriptor/registry.go
@@ -87,6 +87,9 @@ type Registry struct {
 	// has no HttpRule annotation.
 	warnOnUnboundMethods bool
 
+	// proto3OptionalNullable specifies whether Proto3 Optional fields should be marked as x-nullable.
+	proto3OptionalNullable bool
+
 	// fileOptions is a mapping of file name to additional OpenAPI file options
 	fileOptions map[string]*options.Swagger
 
@@ -568,6 +571,16 @@ func (r *Registry) GetOmitPackageDoc() bool {
 	return r.omitPackageDoc
 }
 
+// SetProto3OptionalNullable set proto3OtionalNullable
+func (r *Registry) SetProto3OptionalNullable(proto3OtionalNullable bool) {
+	r.proto3OptionalNullable = proto3OtionalNullable
+}
+
+// GetProto3OptionalNullable returns proto3OtionalNullable
+func (r *Registry) GetProto3OptionalNullable() bool {
+	return r.proto3OptionalNullable
+}
+
 // RegisterOpenAPIOptions registers OpenAPI options
 func (r *Registry) RegisterOpenAPIOptions(opts *openapiconfig.OpenAPIOptions) error {
 	if opts == nil {
diff --git a/protoc-gen-openapiv2/defs.bzl b/protoc-gen-openapiv2/defs.bzl
index ea3226762a5..43596664d55 100644
--- a/protoc-gen-openapiv2/defs.bzl
+++ b/protoc-gen-openapiv2/defs.bzl
@@ -62,6 +62,7 @@ def _run_proto_gen_openapi(
         disable_default_errors,
         enums_as_ints,
         simple_operation_ids,
+        proto3_optional_nullable,
         openapi_configuration,
         generate_unbound_methods):
     args = actions.args()
@@ -107,6 +108,9 @@ def _run_proto_gen_openapi(
     if enums_as_ints:
         args.add("--openapiv2_opt", "enums_as_ints=true")
 
+    if proto3_optional_nullable:
+        args.add("--openapiv2_opt", "proto3_optional_nullable=true")
+
     args.add("--openapiv2_opt", "repeated_path_param_separator=%s" % repeated_path_param_separator)
 
     proto_file_infos = _direct_source_infos(proto_info)
@@ -201,6 +205,7 @@ def _proto_gen_openapi_impl(ctx):
                     disable_default_errors = ctx.attr.disable_default_errors,
                     enums_as_ints = ctx.attr.enums_as_ints,
                     simple_operation_ids = ctx.attr.simple_operation_ids,
+                    proto3_optional_nullable = ctx.attr.proto3_optional_nullable,
                     openapi_configuration = ctx.file.openapi_configuration,
                     generate_unbound_methods = ctx.attr.generate_unbound_methods,
                 ),
@@ -278,6 +283,11 @@ protoc_gen_openapiv2 = rule(
             doc = "whether to remove the service prefix in the operationID" +
                   " generation. Can introduce duplicate operationIDs, use with caution.",
         ),
+        "proto3_optional_nullable": attr.bool(
+            default = False,
+            mandatory = False,
+            doc = "whether Proto3 Optional fields should be marked as x-nullable",
+        ),
         "openapi_configuration": attr.label(
             allow_single_file = True,
             mandatory = False,
diff --git a/protoc-gen-openapiv2/internal/genopenapi/template.go b/protoc-gen-openapiv2/internal/genopenapi/template.go
index 0ac0d64495c..ff0ef171d18 100644
--- a/protoc-gen-openapiv2/internal/genopenapi/template.go
+++ b/protoc-gen-openapiv2/internal/genopenapi/template.go
@@ -410,6 +410,7 @@ func renderMessageAsDefinition(msg *descriptor.Message, reg *descriptor.Registry
 		schema.MaxProperties = protoSchema.MaxProperties
 		schema.MinProperties = protoSchema.MinProperties
 		schema.Required = protoSchema.Required
+		schema.XNullable = protoSchema.XNullable
 		if protoSchema.schemaCore.Type != "" || protoSchema.schemaCore.Ref != "" {
 			schema.schemaCore = protoSchema.schemaCore
 		}
@@ -581,6 +582,10 @@ func schemaOfField(f *descriptor.Field, reg *descriptor.Registry, refs refMap) o
 		updateSwaggerObjectFromFieldBehavior(&ret, j, f)
 	}
 
+	if reg.GetProto3OptionalNullable() && f.GetProto3Optional() {
+		ret.XNullable = true
+	}
+
 	return ret
 }
 
diff --git a/protoc-gen-openapiv2/internal/genopenapi/types.go b/protoc-gen-openapiv2/internal/genopenapi/types.go
index 505cfc686e8..861c4c73338 100644
--- a/protoc-gen-openapiv2/internal/genopenapi/types.go
+++ b/protoc-gen-openapiv2/internal/genopenapi/types.go
@@ -150,12 +150,18 @@ type openapiParameterObject struct {
 }
 
 // core part of schema, which is common to itemsObject and schemaObject.
-// http://swagger.io/specification/#itemsObject
+// http://swagger.io/specification/v2/#itemsObject
+// The OAS3 spec (https://swagger.io/specification/#schemaObject) defines the
+// `nullable` field as part of a Schema Object. This behavior has been
+// "back-ported" to OAS2 as the Specification Extension `x-nullable`, and is
+// supported by generation tools such as swagger-codegen and go-swagger.
+// For protoc-gen-openapiv3, we'd want to add `nullable` instead.
 type schemaCore struct {
-	Type    string          `json:"type,omitempty"`
-	Format  string          `json:"format,omitempty"`
-	Ref     string          `json:"$ref,omitempty"`
-	Example json.RawMessage `json:"example,omitempty"`
+	Type      string          `json:"type,omitempty"`
+	Format    string          `json:"format,omitempty"`
+	Ref       string          `json:"$ref,omitempty"`
+	XNullable bool            `json:"x-nullable,omitempty"`
+	Example   json.RawMessage `json:"example,omitempty"`
 
 	Items *openapiItemsObject `json:"items,omitempty"`
 
diff --git a/protoc-gen-openapiv2/main.go b/protoc-gen-openapiv2/main.go
index b05af0e879d..ff104d9f842 100644
--- a/protoc-gen-openapiv2/main.go
+++ b/protoc-gen-openapiv2/main.go
@@ -31,6 +31,7 @@ var (
 	disableDefaultErrors       = flag.Bool("disable_default_errors", false, "if set, disables generation of default errors. This is useful if you have defined custom error handling")
 	enumsAsInts                = flag.Bool("enums_as_ints", false, "whether to render enum values as integers, as opposed to string values")
 	simpleOperationIDs         = flag.Bool("simple_operation_ids", false, "whether to remove the service prefix in the operationID generation. Can introduce duplicate operationIDs, use with caution.")
+	proto3OptionalNullable     = flag.Bool("proto3_optional_nullable", false, "whether Proto3 Optional fields should be marked as x-nullable")
 	openAPIConfiguration       = flag.String("openapi_configuration", "", "path to file which describes the OpenAPI Configuration in YAML format")
 	generateUnboundMethods     = flag.Bool("generate_unbound_methods", false, "generate swagger metadata even for RPC methods that have no HttpRule annotation")
 	recursiveDepth             = flag.Int("recursive-depth", 1000, "maximum recursion count allowed for a field type")
@@ -89,6 +90,7 @@ func main() {
 	reg.SetEnumsAsInts(*enumsAsInts)
 	reg.SetDisableDefaultErrors(*disableDefaultErrors)
 	reg.SetSimpleOperationIDs(*simpleOperationIDs)
+	reg.SetProto3OptionalNullable(*proto3OptionalNullable)
 	reg.SetGenerateUnboundMethods(*generateUnboundMethods)
 	reg.SetRecursiveDepth(*recursiveDepth)
 	if err := reg.SetRepeatedPathParamSeparator(*repeatedPathParamSeparator); err != nil {