diff --git a/jsonpb/jsonpb.go b/jsonpb/jsonpb.go
index fb5e087..2663c92 100644
--- a/jsonpb/jsonpb.go
+++ b/jsonpb/jsonpb.go
@@ -111,6 +111,18 @@
 			}
 		}
 
+		// Oneof fields need special handling.
+		if valueField.Tag.Get("protobuf_oneof") != "" {
+			// value is an interface containing &T{real_value}.
+			sv := value.Elem().Elem() // interface -> *T -> T
+			value = sv.Field(0)
+			valueField = sv.Type().Field(0)
+
+			var p proto.Properties
+			p.Parse(sv.Type().Field(0).Tag.Get("protobuf"))
+			fieldName = p.OrigName
+		}
+
 		out.write(writeBeforeField)
 		if m.Indent != "" {
 			out.write(indent)
@@ -319,6 +331,44 @@
 				delete(jsonFields, fieldName)
 			}
 		}
+		// Check for any oneof fields.
+		// This might be slow; we can optimise it if it becomes a problem.
+		type oneofMessage interface {
+			XXX_OneofFuncs() (func(proto.Message, *proto.Buffer) error, func(proto.Message, int, int, *proto.Buffer) (bool, error), []interface{})
+		}
+		var oneofTypes []interface{}
+		if om, ok := reflect.Zero(reflect.PtrTo(targetType)).Interface().(oneofMessage); ok {
+			_, _, oneofTypes = om.XXX_OneofFuncs()
+		}
+		for fname, raw := range jsonFields {
+			for _, oot := range oneofTypes {
+				sp := reflect.ValueOf(oot).Type() // *T
+				var props proto.Properties
+				props.Parse(sp.Elem().Field(0).Tag.Get("protobuf"))
+				if props.OrigName != fname {
+					continue
+				}
+				nv := reflect.New(sp.Elem())
+				// There will be exactly one interface field that
+				// this new value is assignable to.
+				for i := 0; i < targetType.NumField(); i++ {
+					f := targetType.Field(i)
+					if f.Type.Kind() != reflect.Interface {
+						continue
+					}
+					if !nv.Type().AssignableTo(f.Type) {
+						continue
+					}
+					target.Field(i).Set(nv)
+					break
+				}
+				if err := unmarshalValue(nv.Elem().Field(0), raw); err != nil {
+					return err
+				}
+				delete(jsonFields, fname)
+				break
+			}
+		}
 		if len(jsonFields) > 0 {
 			// Pick any field to be the scapegoat.
 			var f string