diff --git a/src/libraries/System.Collections/ref/System.Collections.Forwards.cs b/src/libraries/System.Collections/ref/System.Collections.Forwards.cs
new file mode 100644
index 00000000000000..f2d17d02077c43
--- /dev/null
+++ b/src/libraries/System.Collections/ref/System.Collections.Forwards.cs
@@ -0,0 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// ------------------------------------------------------------------------------
+// Changes to this file must follow the https://aka.ms/api-review process.
+// ------------------------------------------------------------------------------
+
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Collections.ObjectModel.ReadOnlySet<>))]
diff --git a/src/libraries/System.Collections/ref/System.Collections.cs b/src/libraries/System.Collections/ref/System.Collections.cs
index b63883803728ab..fc46acb8f17d4b 100644
--- a/src/libraries/System.Collections/ref/System.Collections.cs
+++ b/src/libraries/System.Collections/ref/System.Collections.cs
@@ -582,38 +582,6 @@ void System.Collections.IEnumerator.Reset() { }
         }
     }
 }
-namespace System.Collections.ObjectModel
-{
-    public partial class ReadOnlySet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.IReadOnlySet<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable
-    {
-        public ReadOnlySet(System.Collections.Generic.ISet<T> @set) { }
-        public int Count { get { throw null; } }
-        public static System.Collections.ObjectModel.ReadOnlySet<T> Empty { get { throw null; } }
-        protected System.Collections.Generic.ISet<T> Set { get { throw null; } }
-        bool System.Collections.Generic.ICollection<T>.IsReadOnly { get { throw null; } }
-        bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
-        object System.Collections.ICollection.SyncRoot { get { throw null; } }
-        public bool Contains(T item) { throw null; }
-        public System.Collections.Generic.IEnumerator<T> GetEnumerator() { throw null; }
-        public bool IsProperSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public bool IsProperSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public bool IsSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public bool IsSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public bool Overlaps(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public bool SetEquals(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        void System.Collections.Generic.ICollection<T>.Add(T item) { }
-        void System.Collections.Generic.ICollection<T>.Clear() { }
-        void System.Collections.Generic.ICollection<T>.CopyTo(T[] array, int arrayIndex) { }
-        bool System.Collections.Generic.ICollection<T>.Remove(T item) { throw null; }
-        bool System.Collections.Generic.ISet<T>.Add(T item) { throw null; }
-        void System.Collections.Generic.ISet<T>.ExceptWith(System.Collections.Generic.IEnumerable<T> other) { }
-        void System.Collections.Generic.ISet<T>.IntersectWith(System.Collections.Generic.IEnumerable<T> other) { }
-        void System.Collections.Generic.ISet<T>.SymmetricExceptWith(System.Collections.Generic.IEnumerable<T> other) { }
-        void System.Collections.Generic.ISet<T>.UnionWith(System.Collections.Generic.IEnumerable<T> other) { }
-        void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-    }
-}
 #endif // !BUILDING_CORELIB_REFERENCE
 namespace System.Collections.Generic
 {
@@ -627,6 +595,7 @@ public static void InsertRange<T>(this System.Collections.Generic.List<T> list,
         public static bool Remove<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         public static bool TryAdd<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, TValue value) { throw null; }
         public static System.Collections.ObjectModel.ReadOnlyCollection<T> AsReadOnly<T>(this IList<T> list) { throw null; }
+        public static System.Collections.ObjectModel.ReadOnlySet<T> AsReadOnly<T>(this ISet<T> set) { throw null; }
         public static System.Collections.ObjectModel.ReadOnlyDictionary<TKey, TValue> AsReadOnly<TKey, TValue>(this IDictionary<TKey, TValue> dictionary) where TKey : notnull { throw null; }
     }
     public abstract partial class Comparer<T> : System.Collections.Generic.IComparer<T>, System.Collections.IComparer
diff --git a/src/libraries/System.Collections/ref/System.Collections.csproj b/src/libraries/System.Collections/ref/System.Collections.csproj
index b0c8a034380a17..18d94174306605 100644
--- a/src/libraries/System.Collections/ref/System.Collections.csproj
+++ b/src/libraries/System.Collections/ref/System.Collections.csproj
@@ -4,6 +4,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Collections.cs" />
+    <Compile Include="System.Collections.Forwards.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\System.Runtime\ref\System.Runtime.csproj" />
diff --git a/src/libraries/System.Collections/src/System.Collections.csproj b/src/libraries/System.Collections/src/System.Collections.csproj
index ac718a129a9ad6..8e78d63745bf70 100644
--- a/src/libraries/System.Collections/src/System.Collections.csproj
+++ b/src/libraries/System.Collections/src/System.Collections.csproj
@@ -20,7 +20,6 @@
     <Compile Include="System\Collections\Generic\OrderedDictionary.cs" />
     <Compile Include="System\Collections\Generic\PriorityQueue.cs" />
     <Compile Include="System\Collections\Generic\PriorityQueueDebugView.cs" />
-    <Compile Include="System\Collections\Generic\ReadOnlySet.cs" />
     <Compile Include="System\Collections\Generic\SortedDictionary.cs" />
     <Compile Include="System\Collections\Generic\SortedList.cs" />
     <Compile Include="System\Collections\Generic\SortedSet.cs" />
diff --git a/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs b/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs
index d07e031f0e7acf..013ea47c9b08fd 100644
--- a/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs
+++ b/src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs
@@ -116,6 +116,17 @@ public void AsReadOnly_TurnsIListIntoReadOnlyCollection()
             CollectionAsserts.Equal(list, readOnlyCollection);
         }
 
+        [Fact]
+        public void AsReadOnly_TurnsISetIntoReadOnlySet()
+        {
+            ISet<string> set = new HashSet<string> { "A", "B" };
+            ReadOnlySet<string> readOnlySet = set.AsReadOnly();
+            Assert.NotNull(readOnlySet);
+            Assert.NotSame(set, readOnlySet);
+            Assert.NotSame(readOnlySet, set.AsReadOnly());
+            CollectionAsserts.Equal(set, readOnlySet);
+        }
+
         [Fact]
         public void AsReadOnly_TurnsIDictionaryIntoReadOnlyDictionary()
         {
@@ -134,6 +145,13 @@ public void AsReadOnly_NullIList_ThrowsArgumentNullException()
             Assert.Throws<ArgumentNullException>("list", () => list.AsReadOnly());
         }
 
+        [Fact]
+        public void AsReadOnly_NullISet_ThrowsArgumentNullException()
+        {
+            ISet<string> set = null;
+            AssertExtensions.Throws<ArgumentNullException>("set", () => set.AsReadOnly());
+        }
+
         [Fact]
         public void AsReadOnly_NullIDictionary_ThrowsArgumentNullException()
         {
diff --git a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
index ff2f86e7106faa..3efe4af9fe9a20 100644
--- a/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
+++ b/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems
@@ -243,6 +243,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\CollectionHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\ReadOnlyCollection.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\ReadOnlyDictionary.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Collections\ObjectModel\ReadOnlySet.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\DefaultValueAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\EditorBrowsableAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\ComponentModel\EditorBrowsableState.cs" />
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs
index cf4256f546d59f..6823ab6cfdcaaa 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs
@@ -65,6 +65,17 @@ public static bool Remove<TKey, TValue>(this IDictionary<TKey, TValue> dictionar
         public static ReadOnlyCollection<T> AsReadOnly<T>(this IList<T> list) =>
             new ReadOnlyCollection<T>(list);
 
+        /// <summary>
+        /// Returns a read-only <see cref="ReadOnlySet{T}"/> wrapper
+        /// for the specified set.
+        /// </summary>
+        /// <typeparam name="T">The type of elements in the set.</typeparam>
+        /// <param name="set">The set to wrap.</param>
+        /// <returns>An object that acts as a read-only wrapper around the current <see cref="ISet{T}"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="set"/> is null.</exception>
+        public static ReadOnlySet<T> AsReadOnly<T>(this ISet<T> set) =>
+            new ReadOnlySet<T>(set);
+
         /// <summary>
         /// Returns a read-only <see cref="ReadOnlyDictionary{TKey, TValue}"/> wrapper
         /// for the current dictionary.
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/ReadOnlySet.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlySet.cs
similarity index 100%
rename from src/libraries/System.Collections/src/System/Collections/Generic/ReadOnlySet.cs
rename to src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlySet.cs
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index a08103e2de4a05..4c7a34de961e60 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -8470,6 +8470,35 @@ void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
             System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         }
     }
+    public partial class ReadOnlySet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.IReadOnlySet<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable
+    {
+        public ReadOnlySet(System.Collections.Generic.ISet<T> @set) { }
+        public int Count { get { throw null; } }
+        public static System.Collections.ObjectModel.ReadOnlySet<T> Empty { get { throw null; } }
+        protected System.Collections.Generic.ISet<T> Set { get { throw null; } }
+        bool System.Collections.Generic.ICollection<T>.IsReadOnly { get { throw null; } }
+        bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
+        object System.Collections.ICollection.SyncRoot { get { throw null; } }
+        public bool Contains(T item) { throw null; }
+        public System.Collections.Generic.IEnumerator<T> GetEnumerator() { throw null; }
+        public bool IsProperSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        public bool IsProperSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        public bool IsSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        public bool IsSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        public bool Overlaps(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        public bool SetEquals(System.Collections.Generic.IEnumerable<T> other) { throw null; }
+        void System.Collections.Generic.ICollection<T>.Add(T item) { }
+        void System.Collections.Generic.ICollection<T>.Clear() { }
+        void System.Collections.Generic.ICollection<T>.CopyTo(T[] array, int arrayIndex) { }
+        bool System.Collections.Generic.ICollection<T>.Remove(T item) { throw null; }
+        bool System.Collections.Generic.ISet<T>.Add(T item) { throw null; }
+        void System.Collections.Generic.ISet<T>.ExceptWith(System.Collections.Generic.IEnumerable<T> other) { }
+        void System.Collections.Generic.ISet<T>.IntersectWith(System.Collections.Generic.IEnumerable<T> other) { }
+        void System.Collections.Generic.ISet<T>.SymmetricExceptWith(System.Collections.Generic.IEnumerable<T> other) { }
+        void System.Collections.Generic.ISet<T>.UnionWith(System.Collections.Generic.IEnumerable<T> other) { }
+        void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
+        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
+    }
 }
 namespace System.ComponentModel
 {