diff --git a/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs b/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs
new file mode 100644
index 0000000..09422e7
--- /dev/null
+++ b/PolyShim.Tests/NetCore21/RandomNumberGeneratorTests.cs
@@ -0,0 +1,24 @@
+using System;
+using System.Security.Cryptography;
+using FluentAssertions;
+using Xunit;
+
+namespace PolyShim.Tests.NetCore21;
+
+public class RandomNumberGeneratorTests
+{
+    [Fact]
+    public void Fill_Span_Test()
+    {
+        // Arrange
+        Span<byte> buffer = stackalloc byte[16];
+
+        // Act
+        RandomNumberGenerator.Fill(buffer);
+
+        // Assert
+        // Since cryptographic random data is non-deterministic,
+        // we can only verify that the buffer contains some non-zero bytes
+        buffer.ToArray().Should().Contain(b => b != 0);
+    }
+}
diff --git a/PolyShim/NetCore21/RandomNumberGenerator.cs b/PolyShim/NetCore21/RandomNumberGenerator.cs
new file mode 100644
index 0000000..bd49461
--- /dev/null
+++ b/PolyShim/NetCore21/RandomNumberGenerator.cs
@@ -0,0 +1,25 @@
+#if (NETCOREAPP && !NETCOREAPP2_1_OR_GREATER) || (NETFRAMEWORK && NET45_OR_GREATER)
+#nullable enable
+// ReSharper disable RedundantUsingDirective
+// ReSharper disable CheckNamespace
+// ReSharper disable InconsistentNaming
+// ReSharper disable PartialTypeWithSinglePart
+
+using System;
+using System.Security.Cryptography;
+
+internal static partial class PolyfillExtensions
+{
+    extension(RandomNumberGenerator)
+    {
+        // https://learn.microsoft.com/dotnet/api/system.security.cryptography.randomnumbergenerator.fill
+        public static void Fill(Span<byte> data)
+        {
+            var buffer = new byte[data.Length];
+            using var rng = RandomNumberGenerator.Create();
+            rng.GetBytes(buffer);
+            buffer.CopyTo(data);
+        }
+    }
+}
+#endif