diff --git a/src/Middleware/RateLimiting/src/RateLimiterEndpointConventionBuilderExtensions.cs b/src/Middleware/RateLimiting/src/RateLimiterEndpointConventionBuilderExtensions.cs
index 3fb4d423e047..32d0f3520dc7 100644
--- a/src/Middleware/RateLimiting/src/RateLimiterEndpointConventionBuilderExtensions.cs
+++ b/src/Middleware/RateLimiting/src/RateLimiterEndpointConventionBuilderExtensions.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.Runtime.CompilerServices;
 using Microsoft.AspNetCore.RateLimiting;
 
 namespace Microsoft.AspNetCore.Builder;
@@ -40,9 +41,18 @@ public static TBuilder RequireRateLimiting<TBuilder, TPartitionKey>(this TBuilde
         ArgumentNullException.ThrowIfNull(builder);
         ArgumentNullException.ThrowIfNull(policy);
 
+        // Inline policies are not named, so historically they all shared a single null partition-key
+        // namespace. That let two distinct inline policies collide onto the same limiter whenever their
+        // GetPartition methods returned equal keys. Derive a stable namespace from the policy instance so
+        // distinct policy objects are isolated while the same instance reused across endpoints still shares
+        // a limiter. This is captured once here rather than inside the convention callback so endpoint
+        // rebuilds don't regenerate it. It is intentionally kept separate from the telemetry-facing
+        // EnableRateLimitingAttribute.PolicyName so rate-limiting metrics are unaffected.
+        var policyName = $"__inlinePolicy_{RuntimeHelpers.GetHashCode(policy)}";
+
         builder.Add(endpointBuilder =>
         {
-            endpointBuilder.Metadata.Add(new EnableRateLimitingAttribute(new DefaultRateLimiterPolicy(RateLimiterOptions.ConvertPartitioner<TPartitionKey>(null, policy.GetPartition), policy.OnRejected)));
+            endpointBuilder.Metadata.Add(new EnableRateLimitingAttribute(new DefaultRateLimiterPolicy(RateLimiterOptions.ConvertPartitioner<TPartitionKey>(policyName, policy.GetPartition), policy.OnRejected)));
         });
         return builder;
     }
diff --git a/src/Middleware/RateLimiting/test/RateLimitingMiddlewareTests.cs b/src/Middleware/RateLimiting/test/RateLimitingMiddlewareTests.cs
index 2792d51d6797..a44985b12aa3 100644
--- a/src/Middleware/RateLimiting/test/RateLimitingMiddlewareTests.cs
+++ b/src/Middleware/RateLimiting/test/RateLimitingMiddlewareTests.cs
@@ -579,6 +579,61 @@ public async Task PolicyDirectlyOnEndpoint_GetsUsed()
         Assert.Equal(StatusCodes.Status404NotFound, context.Response.StatusCode);
     }
 
+    [Fact]
+    public async Task EndpointLimiter_InlinePolicy_DuplicatePartitionKey_NoCollision()
+    {
+        // Two distinct inline policy instances that return the same partition key must not share a
+        // limiter. Before the fix both inline policies used a null partition-key namespace, so whichever
+        // endpoint ran first created the shared limiter and the other silently reused it.
+        var options = CreateOptionsAccessor();
+        var duplicateKey = "myKey";
+        var policy1 = new CountingRateLimiterPolicy(duplicateKey, permitLimit: 1);
+        var policy2 = new CountingRateLimiterPolicy(duplicateKey, permitLimit: 1);
+
+        var middleware = CreateTestRateLimitingMiddleware(options);
+
+        var endpoint1 = CreateEndpointWithRateLimitPolicy(policy1);
+        var endpoint2 = CreateEndpointWithRateLimitPolicy(policy2);
+
+        // Exhaust policy1's single permit on endpoint1.
+        var context = new DefaultHttpContext();
+        context.SetEndpoint(endpoint1);
+        await middleware.Invoke(context).DefaultTimeout();
+        Assert.Equal(StatusCodes.Status200OK, context.Response.StatusCode);
+
+        // endpoint2 uses a different policy instance, so it has its own limiter and its own permit.
+        context = new DefaultHttpContext();
+        context.SetEndpoint(endpoint2);
+        await middleware.Invoke(context).DefaultTimeout();
+        Assert.Equal(StatusCodes.Status200OK, context.Response.StatusCode);
+    }
+
+    [Fact]
+    public async Task EndpointLimiter_InlinePolicy_SameInstance_SharesLimiter()
+    {
+        // The same inline policy instance reused across endpoints should keep sharing a single limiter,
+        // so per-instance identity (not per-registration) is used for the partition-key namespace.
+        var options = CreateOptionsAccessor();
+        var policy = new CountingRateLimiterPolicy("myKey", permitLimit: 1);
+
+        var middleware = CreateTestRateLimitingMiddleware(options);
+
+        var endpoint1 = CreateEndpointWithRateLimitPolicy(policy);
+        var endpoint2 = CreateEndpointWithRateLimitPolicy(policy);
+
+        // First endpoint consumes the single shared permit.
+        var context = new DefaultHttpContext();
+        context.SetEndpoint(endpoint1);
+        await middleware.Invoke(context).DefaultTimeout();
+        Assert.Equal(StatusCodes.Status200OK, context.Response.StatusCode);
+
+        // Second endpoint shares the same limiter, so no permit remains and it is rejected.
+        context = new DefaultHttpContext();
+        context.SetEndpoint(endpoint2);
+        await middleware.Invoke(context).DefaultTimeout();
+        Assert.Equal(StatusCodes.Status503ServiceUnavailable, context.Response.StatusCode);
+    }
+
     [Fact]
     public async Task MultipleEndpointPolicies_LastOneWins()
     {
@@ -654,4 +709,51 @@ private RateLimitingMiddleware CreateTestRateLimitingMiddleware(IOptions<RateLim
             new RateLimitingMetrics(new TestMeterFactory()));
 
     private IOptions<RateLimiterOptions> CreateOptionsAccessor() => Options.Create(new RateLimiterOptions());
+
+    // An inline policy backed by a stateful limiter that permits a fixed number of acquisitions and then
+    // rejects. Used to observe whether two endpoints share or isolate their limiter buckets.
+    private sealed class CountingRateLimiterPolicy : IRateLimiterPolicy<string>
+    {
+        private readonly string _key;
+        private readonly int _permitLimit;
+
+        public CountingRateLimiterPolicy(string key, int permitLimit)
+        {
+            _key = key;
+            _permitLimit = permitLimit;
+        }
+
+        public Func<OnRejectedContext, CancellationToken, ValueTask> OnRejected => null;
+
+        public RateLimitPartition<string> GetPartition(HttpContext httpContext)
+            => RateLimitPartition.Get(_key, _ => new CountingRateLimiter(_permitLimit));
+    }
+
+    private sealed class CountingRateLimiter : RateLimiter
+    {
+        private int _remaining;
+
+        public CountingRateLimiter(int permitLimit) => _remaining = permitLimit;
+
+        public override TimeSpan? IdleDuration => null;
+
+        public override RateLimiterStatistics GetStatistics() => null;
+
+        protected override RateLimitLease AttemptAcquireCore(int permitCount)
+        {
+            if (_remaining >= permitCount)
+            {
+                _remaining -= permitCount;
+                return new TestRateLimitLease(isAcquired: true, null);
+            }
+
+            return new TestRateLimitLease(isAcquired: false, null);
+        }
+
+        protected override ValueTask<RateLimitLease> AcquireAsyncCore(int permitCount, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            return new ValueTask<RateLimitLease>(AttemptAcquireCore(permitCount));
+        }
+    }
 }