diff --git a/src/libraries/System.Private.CoreLib/src/System/OperatingSystem.cs b/src/libraries/System.Private.CoreLib/src/System/OperatingSystem.cs
index 26fbb1510a0c3..6fc2f2257fe7b 100644
--- a/src/libraries/System.Private.CoreLib/src/System/OperatingSystem.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/OperatingSystem.cs
@@ -150,7 +150,6 @@ public static bool IsBrowser() =>
             false;
 #endif
 
-/* TODO https://github.com/dotnet/runtime/issues/78389
         /// <summary>
         /// Indicates whether the current application is running as WASI.
         /// </summary>
@@ -161,7 +160,6 @@ public static bool IsWasi() =>
 #else
             false;
 #endif
-*/
 
         /// <summary>
         /// Indicates whether the current application is running on Linux.
diff --git a/src/libraries/System.Runtime.Extensions/tests/System/OperatingSystemTests.cs b/src/libraries/System.Runtime.Extensions/tests/System/OperatingSystemTests.cs
index 3a7e5aaa60f25..adaf447f1fc37 100644
--- a/src/libraries/System.Runtime.Extensions/tests/System/OperatingSystemTests.cs
+++ b/src/libraries/System.Runtime.Extensions/tests/System/OperatingSystemTests.cs
@@ -18,7 +18,8 @@ public static class OperatingSystemTests
             "Windows",
             "Linux",
             "FreeBSD",
-            "Browser"
+            "Browser",
+            "Wasi",
         };
 
         [Theory]
@@ -80,6 +81,12 @@ public static void IsOSPlatformVersionAtLeast_InvalidArgs_Throws()
         [Fact, PlatformSpecific(TestPlatforms.Browser)]
         public static void TestIsOSVersionAtLeast_Browser() => TestIsOSVersionAtLeast("BROWSER");
 
+        [Fact, PlatformSpecific(TestPlatforms.Wasi)]
+        public static void TestIsOSPlatform_Wasi() => TestIsOSPlatform("WASI", OperatingSystem.IsWasi);
+
+        [Fact, PlatformSpecific(TestPlatforms.Wasi)]
+        public static void TestIsOSVersionAtLeast_Wasi() => TestIsOSVersionAtLeast("WASI");
+
         [Fact, PlatformSpecific(TestPlatforms.Linux)]
         public static void TestIsOSPlatform_Linux() => TestIsOSPlatform("Linux", OperatingSystem.IsLinux);
 
@@ -191,7 +198,8 @@ private static void TestIsOSPlatform(string currentOSName, Func<bool> currentOSC
                 OperatingSystem.IsMacOS(),
                 OperatingSystem.IsTvOS(),
                 OperatingSystem.IsWatchOS(),
-                OperatingSystem.IsWindows()
+                OperatingSystem.IsWindows(),
+                OperatingSystem.IsWasi(),
             };
 
             // MacCatalyst is a special case since it also returns true for iOS
@@ -208,6 +216,7 @@ private static void TestIsOSPlatform(string currentOSName, Func<bool> currentOSC
                 Assert.False(allResults[7]); // IsTvOS()
                 Assert.False(allResults[8]); // IsWatchOS()
                 Assert.False(allResults[9]); // IsWindows()
+                Assert.False(allResults[10]); // IsWasi()
             }
             else
             {
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index 2de21393bbc2b..318cb39b988f5 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -4334,6 +4334,7 @@ public void GetObjectData(System.Runtime.Serialization.SerializationInfo info, S
         public static bool IsAndroid() { throw null; }
         public static bool IsAndroidVersionAtLeast(int major, int minor = 0, int build = 0, int revision = 0) { throw null; }
         public static bool IsBrowser() { throw null; }
+        public static bool IsWasi() { throw null; }
         public static bool IsFreeBSD() { throw null; }
         public static bool IsFreeBSDVersionAtLeast(int major, int minor = 0, int build = 0, int revision = 0) { throw null; }
         [System.Runtime.Versioning.SupportedOSPlatformGuardAttribute("maccatalyst")]