diff --git a/encoding/jsonpb/decode.go b/encoding/jsonpb/decode.go
index 7655054..3d4a068 100644
--- a/encoding/jsonpb/decode.go
+++ b/encoding/jsonpb/decode.go
@@ -153,6 +153,7 @@
 	var nerr errors.NonFatal
 	var reqNums set.Ints
 	var seenNums set.Ints
+	var seenOneofs set.Ints
 
 	msgType := m.Type()
 	knownFields := m.KnownFields()
@@ -237,6 +238,15 @@
 				return errors.New("%v|%q: %v", fd.FullName(), name, err)
 			}
 		} else {
+			// If field is a oneof, check if it has already been set.
+			if od := fd.OneofType(); od != nil {
+				idx := uint64(od.Index())
+				if seenOneofs.Has(idx) {
+					return errors.New("%v: oneof is already set", od.FullName())
+				}
+				seenOneofs.Set(idx)
+			}
+
 			// Required or optional fields.
 			if err := o.unmarshalSingular(knownFields, fd); !nerr.Merge(err) {
 				return errors.New("%v|%q: %v", fd.FullName(), name, err)
diff --git a/encoding/jsonpb/decode_test.go b/encoding/jsonpb/decode_test.go
index 587b2b4..9d50ac1 100644
--- a/encoding/jsonpb/decode_test.go
+++ b/encoding/jsonpb/decode_test.go
@@ -678,6 +678,26 @@
 			},
 		},
 	}, {
+		desc:         "oneof set to more than one field",
+		inputMessage: &pb3.Oneofs{},
+		inputText: `{
+  "oneofEnum": "ZERO",
+  "oneofString": "hello"
+}`,
+		wantErr: true,
+	}, {
+		desc:         "oneof set to null and value",
+		inputMessage: &pb3.Oneofs{},
+		inputText: `{
+  "oneofEnum": "ZERO",
+  "oneofString": null
+}`,
+		wantMessage: &pb3.Oneofs{
+			Union: &pb3.Oneofs_OneofEnum{
+				OneofEnum: pb3.Enum_ZERO,
+			},
+		},
+	}, {
 		desc:         "repeated null fields",
 		inputMessage: &pb2.Repeats{},
 		inputText: `{
diff --git a/encoding/textpb/decode.go b/encoding/textpb/decode.go
index ae2015a..38b512e 100644
--- a/encoding/textpb/decode.go
+++ b/encoding/textpb/decode.go
@@ -104,6 +104,7 @@
 	xtTypes := knownFields.ExtensionTypes()
 	var reqNums set.Ints
 	var seenNums set.Ints
+	var seenOneofs set.Ints
 
 	for _, tfield := range tmsg {
 		tkey := tfield[0]
@@ -158,6 +159,15 @@
 				return err
 			}
 		} else {
+			// If field is a oneof, check if it has already been set.
+			if od := fd.OneofType(); od != nil {
+				idx := uint64(od.Index())
+				if seenOneofs.Has(idx) {
+					return errors.New("oneof %v is already set", od.FullName())
+				}
+				seenOneofs.Set(idx)
+			}
+
 			// Required or optional fields.
 			num := uint64(fd.Number())
 			if seenNums.Has(num) {
diff --git a/encoding/textpb/decode_test.go b/encoding/textpb/decode_test.go
index 2f4045c..2f14029 100644
--- a/encoding/textpb/decode_test.go
+++ b/encoding/textpb/decode_test.go
@@ -523,20 +523,13 @@
 			},
 		},
 	}, {
-		desc:         "oneof set to last value",
+		desc:         "oneof set to more than one field",
 		inputMessage: &pb3.Oneofs{},
 		inputText: `
-oneof_nested: {
-  s_string: "nested message"
-}
-oneof_enum: TEN
-oneof_string: "wins"
+oneof_enum: ZERO
+oneof_string: "hello"
 `,
-		wantMessage: &pb3.Oneofs{
-			Union: &pb3.Oneofs_OneofString{
-				OneofString: "wins",
-			},
-		},
+		wantErr: true,
 	}, {
 		desc:         "repeated scalar using same field name",
 		inputMessage: &pb2.Repeats{},