diff --git a/src/Middleware/Rewrite/src/PatternSegments/RewriteMapSegment.cs b/src/Middleware/Rewrite/src/PatternSegments/RewriteMapSegment.cs
index b98be41adbdb..1fb9df0e1efe 100644
--- a/src/Middleware/Rewrite/src/PatternSegments/RewriteMapSegment.cs
+++ b/src/Middleware/Rewrite/src/PatternSegments/RewriteMapSegment.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Text;
 using Microsoft.AspNetCore.Rewrite.IISUrlRewrite;
 
 namespace Microsoft.AspNetCore.Rewrite.PatternSegments;
@@ -18,7 +19,12 @@ public RewriteMapSegment(IISRewriteMap rewriteMap, Pattern pattern)
 
     public override string? Evaluate(RewriteContext context, BackReferenceCollection? ruleBackReferences, BackReferenceCollection? conditionBackReferences)
     {
+        // PERF as we share the string builder across the context, we need to make a new one here to
+        // evaluate the rewrite map key, which may itself contain nested pattern segments.
+        var tempBuilder = context.Builder;
+        context.Builder = new StringBuilder(64);
         var key = _pattern.Evaluate(context, ruleBackReferences, conditionBackReferences).ToLowerInvariant();
+        context.Builder = tempBuilder;
         return _rewriteMap[key];
     }
 }
diff --git a/src/Middleware/Rewrite/test/IISUrlRewrite/MiddleWareTests.cs b/src/Middleware/Rewrite/test/IISUrlRewrite/MiddleWareTests.cs
index 6c83b25b860c..ad572ab854a9 100644
--- a/src/Middleware/Rewrite/test/IISUrlRewrite/MiddleWareTests.cs
+++ b/src/Middleware/Rewrite/test/IISUrlRewrite/MiddleWareTests.cs
@@ -150,6 +150,51 @@ public async Task Invoke_RewriteBasedOnQueryStringParameters()
         Assert.Equal("/newpage.aspx?param1=123&param2=321", response);
     }
 
+    [Fact]
+    // Regression test for https://github.com/dotnet/aspnetcore/issues/24618. A rewrite map lookup
+    // nested inside an action URL (for example {id-map:{C:1}}) must evaluate its key with its own
+    // string builder. Otherwise the key evaluation appends to and clears the shared RewriteContext
+    // builder that the surrounding pattern is still using, so the map lookup misses and the rest of
+    // the action URL is lost.
+    public async Task Invoke_RewriteMapNestedInActionUrlEvaluatesKey()
+    {
+        var options = new RewriteOptions().AddIISUrlRewrite(new StringReader(@"<rewrite>
+                <rules>
+                <rule name=""Rewrite with rewrite map"" stopProcessing=""true"">
+                <match url=""page\.asp$"" />
+                <conditions>
+                <add input=""{QUERY_STRING}"" pattern=""id=([0-9-]+)"" />
+                </conditions>
+                <action type=""Rewrite"" url=""newpage.aspx?id={id-map:{C:1}}"" appendQueryString=""false"" />
+                </rule>
+                </rules>
+                <rewriteMaps>
+                <rewriteMap name=""id-map"" defaultValue="""">
+                <add key=""1234-1234"" value=""abcd-abcd"" />
+                </rewriteMap>
+                </rewriteMaps>
+                </rewrite>"));
+        using var host = new HostBuilder()
+            .ConfigureWebHost(webHostBuilder =>
+            {
+                webHostBuilder
+                .UseTestServer()
+                .Configure(app =>
+                {
+                    app.UseRewriter(options);
+                    app.Run(context => context.Response.WriteAsync(context.Request.Path + context.Request.QueryString));
+                });
+            }).Build();
+
+        await host.StartAsync();
+
+        var server = host.GetTestServer();
+
+        var response = await server.CreateClient().GetStringAsync("page.asp?id=1234-1234");
+
+        Assert.Equal("/newpage.aspx?id=abcd-abcd", response);
+    }
+
     [Fact]
     public async Task Invoke_RedirectToLowerCase()
     {