diff --git a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
index 22c861326..e9aca10ac 100644
--- a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
+++ b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
@@ -23,8 +23,21 @@ internal sealed class StreamableHttpHandler(
     ILoggerFactory loggerFactory)
 {
     private const string McpSessionIdHeaderName = "Mcp-Session-Id";
+    private const string McpProtocolVersionHeaderName = "MCP-Protocol-Version";
     private const string LastEventIdHeaderName = "Last-Event-ID";
 
+    /// <summary>
+    /// All protocol versions supported by this implementation.
+    /// Keep in sync with McpSessionHandler.SupportedProtocolVersions in ModelContextProtocol.Core.
+    /// </summary>
+    private static readonly HashSet<string> s_supportedProtocolVersions =
+    [
+        "2024-11-05",
+        "2025-03-26",
+        "2025-06-18",
+        "2025-11-25",
+    ];
+
     private static readonly JsonTypeInfo<JsonRpcMessage> s_messageTypeInfo = GetRequiredJsonTypeInfo<JsonRpcMessage>();
     private static readonly JsonTypeInfo<JsonRpcError> s_errorTypeInfo = GetRequiredJsonTypeInfo<JsonRpcError>();
 
@@ -32,6 +45,12 @@ internal sealed class StreamableHttpHandler(
 
     public async Task HandlePostRequestAsync(HttpContext context)
     {
+        if (!ValidateProtocolVersionHeader(context, out var errorMessage))
+        {
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
+            return;
+        }
+
         // The Streamable HTTP spec mandates the client MUST accept both application/json and text/event-stream.
         // ASP.NET Core Minimal APIs mostly try to stay out of the business of response content negotiation,
         // so we have to do this manually. The spec doesn't mandate that servers MUST reject these requests,
@@ -74,6 +93,12 @@ await WriteJsonRpcErrorAsync(context,
 
     public async Task HandleGetRequestAsync(HttpContext context)
     {
+        if (!ValidateProtocolVersionHeader(context, out var errorMessage))
+        {
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
+            return;
+        }
+
         if (!context.Request.GetTypedHeaders().Accept.Any(MatchesTextEventStreamMediaType))
         {
             await WriteJsonRpcErrorAsync(context,
@@ -171,6 +196,12 @@ private static async Task HandleResumePostResponseStreamAsync(HttpContext contex
 
     public async Task HandleDeleteRequestAsync(HttpContext context)
     {
+        if (!ValidateProtocolVersionHeader(context, out var errorMessage))
+        {
+            await WriteJsonRpcErrorAsync(context, errorMessage!, StatusCodes.Status400BadRequest);
+            return;
+        }
+
         var sessionId = context.Request.Headers[McpSessionIdHeaderName].ToString();
         if (sessionManager.TryRemove(sessionId, out var session))
         {
@@ -390,6 +421,24 @@ internal static Task RunSessionAsync(HttpContext httpContext, McpServer session,
 
     internal static JsonTypeInfo<T> GetRequiredJsonTypeInfo<T>() => (JsonTypeInfo<T>)McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(T));
 
+    /// <summary>
+    /// Validates the MCP-Protocol-Version header if present. A missing header is allowed for backwards compatibility,
+    /// but an invalid or unsupported value must be rejected with 400 Bad Request per the MCP spec.
+    /// </summary>
+    private static bool ValidateProtocolVersionHeader(HttpContext context, out string? errorMessage)
+    {
+        var protocolVersionHeader = context.Request.Headers[McpProtocolVersionHeaderName].ToString();
+        if (!string.IsNullOrEmpty(protocolVersionHeader) &&
+            !s_supportedProtocolVersions.Contains(protocolVersionHeader))
+        {
+            errorMessage = $"Bad Request: The MCP-Protocol-Version header value '{protocolVersionHeader}' is not supported.";
+            return false;
+        }
+
+        errorMessage = null;
+        return true;
+    }
+
     private static bool MatchesApplicationJsonMediaType(MediaTypeHeaderValue acceptHeaderValue)
         => acceptHeaderValue.MatchesMediaType("application/json");
 
diff --git a/src/ModelContextProtocol.Core/McpSessionHandler.cs b/src/ModelContextProtocol.Core/McpSessionHandler.cs
index 9137218eb..1aa444692 100644
--- a/src/ModelContextProtocol.Core/McpSessionHandler.cs
+++ b/src/ModelContextProtocol.Core/McpSessionHandler.cs
@@ -31,7 +31,10 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
     /// <summary>The latest version of the protocol supported by this implementation.</summary>
     internal const string LatestProtocolVersion = "2025-11-25";
 
-    /// <summary>All protocol versions supported by this implementation.</summary>
+    /// <summary>
+    /// All protocol versions supported by this implementation.
+    /// Keep in sync with s_supportedProtocolVersions in StreamableHttpHandler.
+    /// </summary>
     internal static readonly string[] SupportedProtocolVersions =
     [
         "2024-11-05",
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
index 0c42b3e82..a667302df 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
+++ b/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
@@ -158,6 +158,54 @@ public async Task GetRequest_IsAcceptable_WithWildcardOrAddedQualityInAcceptHead
         Assert.Equal(HttpStatusCode.OK, response.StatusCode);
     }
 
+    [Theory]
+    [InlineData("invalid-version")]
+    [InlineData("9999-01-01")]
+    [InlineData("not-a-date")]
+    public async Task PostRequest_IsBadRequest_WithInvalidProtocolVersionHeader(string invalidVersion)
+    {
+        await StartAsync();
+
+        HttpClient.DefaultRequestHeaders.Add("MCP-Protocol-Version", invalidVersion);
+
+        using var response = await HttpClient.PostAsync("", JsonContent(InitializeRequest), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task PostRequest_Succeeds_WithoutProtocolVersionHeader()
+    {
+        await StartAsync();
+
+        // No MCP-Protocol-Version header is set - this should be accepted for backwards compatibility
+        using var response = await HttpClient.PostAsync("", JsonContent(InitializeRequest), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task PostRequest_Succeeds_WithValidProtocolVersionHeader()
+    {
+        await StartAsync();
+
+        HttpClient.DefaultRequestHeaders.Add("MCP-Protocol-Version", "2025-03-26");
+
+        using var response = await HttpClient.PostAsync("", JsonContent(InitializeRequest), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task GetRequest_IsBadRequest_WithInvalidProtocolVersionHeader()
+    {
+        await StartAsync();
+
+        await CallInitializeAndValidateAsync();
+
+        HttpClient.DefaultRequestHeaders.Add("MCP-Protocol-Version", "invalid-version");
+
+        using var response = await HttpClient.GetAsync("", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+    }
+
     [Fact]
     public async Task PostRequest_IsNotFound_WithUnrecognizedSessionId()
     {