diff --git a/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs b/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs
index 0aea60fd1921e0..5148ca0c488d80 100644
--- a/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs
+++ b/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/RuntimeHelpersTests.cs
@@ -394,7 +394,6 @@ public static void ArrayGetSubArrayCoVarianceTest()
         }
 
         [Fact]
-        [SkipOnMono("Not presently implemented on Mono")]
         public static void AllocateTypeAssociatedMemoryInvalidArguments()
         {
             Assert.Throws<ArgumentException>(() => { RuntimeHelpers.AllocateTypeAssociatedMemory(null, 10); });
@@ -402,7 +401,6 @@ public static void AllocateTypeAssociatedMemoryInvalidArguments()
         }
 
         [Fact]
-        [SkipOnMono("Not presently implemented on Mono")]
         public static unsafe void AllocateTypeAssociatedMemoryValidArguments()
         {
             IntPtr memory = RuntimeHelpers.AllocateTypeAssociatedMemory(typeof(RuntimeHelpersTests), 32);
diff --git a/src/mono/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.Mono.cs b/src/mono/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.Mono.cs
index 391bfaec6a2a46..ca9ecdbe9f829c 100644
--- a/src/mono/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.Mono.cs
+++ b/src/mono/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.Mono.cs
@@ -4,6 +4,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Tracing;
+using System.Runtime.InteropServices;
 using System.Runtime.Serialization;
 
 namespace System.Runtime.CompilerServices
@@ -137,9 +138,15 @@ public static void RunModuleConstructor(ModuleHandle module)
             RunModuleConstructor(module.Value);
         }
 
-        public static IntPtr AllocateTypeAssociatedMemory(Type type, int size)
+        public static unsafe IntPtr AllocateTypeAssociatedMemory(Type type, int size)
         {
-            throw new PlatformNotSupportedException();
+            if (type is not RuntimeType)
+                throw new ArgumentException(SR.Arg_MustBeType, nameof(type));
+
+            ArgumentOutOfRangeException.ThrowIfNegative(size);
+
+            // We don't support unloading; the memory will never be freed.
+            return (IntPtr)NativeMemory.AllocZeroed((uint)size);
         }
 
         [Intrinsic]