diff --git a/internal/converter/testdata/wellknown.go b/internal/converter/testdata/wellknown.go
index 92f23a8f..1fc75eb8 100644
--- a/internal/converter/testdata/wellknown.go
+++ b/internal/converter/testdata/wellknown.go
@@ -28,9 +28,10 @@ const WellKnown = `{
             "type": "array"
         },
         "duration": {
-            "additionalProperties": true,
+            "pattern": "^([0-9]+\\.?[0-9]*|\\.[0-9]+)s$",
             "type": "string",
-            "description": "This is a duration:"
+            "description": "This is a duration:",
+            "format": "regex"
         }
     },
     "additionalProperties": true,
diff --git a/internal/converter/types.go b/internal/converter/types.go
index 7d369570..dc2cddfe 100644
--- a/internal/converter/types.go
+++ b/internal/converter/types.go
@@ -181,6 +181,11 @@ func (c *Converter) convertField(curPkg *ProtoPackage, desc *descriptor.FieldDes
 
 	case descriptor.FieldDescriptorProto_TYPE_GROUP, descriptor.FieldDescriptorProto_TYPE_MESSAGE:
 		switch desc.GetTypeName() {
+		// Make sure that durations match a particular string pattern (eg 3.4s):
+		case ".google.protobuf.Duration":
+			jsonSchemaType.Type = gojsonschema.TYPE_STRING
+			jsonSchemaType.Format = "regex"
+			jsonSchemaType.Pattern = `^([0-9]+\.?[0-9]*|\.[0-9]+)s$`
 		case ".google.protobuf.Timestamp":
 			jsonSchemaType.Type = gojsonschema.TYPE_STRING
 			jsonSchemaType.Format = "date-time"