diff --git a/src/IceRpc.Protobuf/RpcMethods/Internal/PipeReaderExtensions.cs b/src/IceRpc.Protobuf/RpcMethods/Internal/PipeReaderExtensions.cs
index 86ce27f48a..5e03495283 100644
--- a/src/IceRpc.Protobuf/RpcMethods/Internal/PipeReaderExtensions.cs
+++ b/src/IceRpc.Protobuf/RpcMethods/Internal/PipeReaderExtensions.cs
@@ -31,6 +31,23 @@ internal static async ValueTask<T> DecodeProtobufMessageAsync<T>(
             cancellationToken).ConfigureAwait(false);
 
         Debug.Assert(message is not null);
+
+        // A unary payload must contain exactly one message; any trailing bytes indicate a framing error.
+        ReadResult readResult = await reader.ReadAsync(cancellationToken).ConfigureAwait(false);
+        // We never call CancelPendingRead; an interceptor or middleware can but it's not correct.
+        if (readResult.IsCanceled)
+        {
+            throw new InvalidOperationException("Unexpected call to CancelPendingRead.");
+        }
+        bool hasTrailingBytes = !readResult.Buffer.IsEmpty;
+        reader.AdvanceTo(readResult.Buffer.End);
+        if (hasTrailingBytes)
+        {
+            throw new InvalidDataException(
+                "The payload contains unexpected trailing bytes after the Protobuf message.");
+        }
+        Debug.Assert(readResult.IsCompleted);
+
         return message;
     }
 
diff --git a/tests/IceRpc.Protobuf.Tests/PipeReaderExtensionsTests.cs b/tests/IceRpc.Protobuf.Tests/PipeReaderExtensionsTests.cs
index 26a6704204..2aaff7d7a2 100644
--- a/tests/IceRpc.Protobuf.Tests/PipeReaderExtensionsTests.cs
+++ b/tests/IceRpc.Protobuf.Tests/PipeReaderExtensionsTests.cs
@@ -1,8 +1,11 @@
 // Copyright (c) ZeroC, Inc.
 
+using Google.Protobuf;
 using Google.Protobuf.WellKnownTypes;
 using IceRpc.Protobuf.RpcMethods.Internal;
 using NUnit.Framework;
+using System.Buffers;
+using System.Buffers.Binary;
 using System.IO.Pipelines;
 
 namespace IceRpc.Protobuf.Tests;
@@ -26,4 +29,49 @@ await pipeReader.DecodeProtobufMessageAsync(
                 CancellationToken.None));
         pipeReader.Complete();
     }
+
+    [Test]
+    public void Decode_message_throws_invalid_data_exception_when_payload_has_trailing_bytes()
+    {
+        // Arrange
+        var pipe = new Pipe();
+        WriteLengthPrefixedMessage(pipe.Writer, new StringValue { Value = "hello" });
+        pipe.Writer.Write(new byte[] { 0xDE, 0xAD, 0xBE, 0xEF });
+        pipe.Writer.Complete();
+
+        // Act & Assert
+        Assert.ThrowsAsync<InvalidDataException>(async () =>
+            await pipe.Reader.DecodeProtobufMessageAsync(
+                StringValue.Parser,
+                maxMessageLength: 1024,
+                CancellationToken.None));
+        pipe.Reader.Complete();
+    }
+
+    [Test]
+    public void Decode_message_throws_invalid_data_exception_when_payload_has_concatenated_messages()
+    {
+        // Arrange
+        var pipe = new Pipe();
+        WriteLengthPrefixedMessage(pipe.Writer, new StringValue { Value = "hello" });
+        WriteLengthPrefixedMessage(pipe.Writer, new StringValue { Value = "world" });
+        pipe.Writer.Complete();
+
+        // Act & Assert
+        Assert.ThrowsAsync<InvalidDataException>(async () =>
+            await pipe.Reader.DecodeProtobufMessageAsync(
+                StringValue.Parser,
+                maxMessageLength: 1024,
+                CancellationToken.None));
+        pipe.Reader.Complete();
+    }
+
+    private static void WriteLengthPrefixedMessage(PipeWriter writer, IMessage message)
+    {
+        writer.Write(new byte[] { 0 }); // Not compressed
+        Span<byte> lengthBytes = writer.GetSpan(4);
+        BinaryPrimitives.WriteInt32BigEndian(lengthBytes, message.CalculateSize());
+        writer.Advance(4);
+        message.WriteTo(writer);
+    }
 }