diff --git a/src/mscorlib/src/System/Text/ASCIIEncoding.cs b/src/mscorlib/src/System/Text/ASCIIEncoding.cs
index 307e4899b706..34b04a0a60ab 100644
--- a/src/mscorlib/src/System/Text/ASCIIEncoding.cs
+++ b/src/mscorlib/src/System/Text/ASCIIEncoding.cs
@@ -23,9 +23,13 @@ namespace System.Text
     [Serializable]
     public class ASCIIEncoding : Encoding
     {
+        // Allow for devirtualization (see https://github.com/dotnet/coreclr/pull/9230)
+        [Serializable]
+        internal sealed class ASCIIEncodingSealed : ASCIIEncoding { }
+
         // Used by Encoding.ASCII for lazy initialization
         // The initialization code will not be run until a static member of the class is referenced
-        internal static readonly ASCIIEncoding s_default = new ASCIIEncoding();
+        internal static readonly ASCIIEncodingSealed s_default = new ASCIIEncodingSealed();
 
         public ASCIIEncoding() : base(Encoding.CodePageASCII)
         {
diff --git a/src/mscorlib/src/System/Text/UTF8Encoding.cs b/src/mscorlib/src/System/Text/UTF8Encoding.cs
index b2cc462027f8..65d9bf9d5e1e 100644
--- a/src/mscorlib/src/System/Text/UTF8Encoding.cs
+++ b/src/mscorlib/src/System/Text/UTF8Encoding.cs
@@ -53,9 +53,16 @@ public class UTF8Encoding : Encoding
 
         private const int UTF8_CODEPAGE = 65001;
 
+        // Allow for devirtualization (see https://github.com/dotnet/coreclr/pull/9230)
+        [Serializable]
+        internal sealed class UTF8EncodingSealed : UTF8Encoding
+        {
+            public UTF8EncodingSealed() : base(encoderShouldEmitUTF8Identifier: true) { }
+        }
+
         // Used by Encoding.UTF8 for lazy initialization
         // The initialization code will not be run until a static member of the class is referenced
-        internal static readonly UTF8Encoding s_default = new UTF8Encoding(encoderShouldEmitUTF8Identifier: true);
+        internal static readonly UTF8EncodingSealed s_default = new UTF8EncodingSealed();
 
         // Yes, the idea of emitting U+FEFF as a UTF-8 identifier has made it into
         // the standard.