diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs b/src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs
index 58efa18407c70..dd0263133846f 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/StreamWriter.cs
@@ -77,7 +77,7 @@ public StreamWriter(Stream stream)
         {
         }
 
-        public StreamWriter(Stream stream, Encoding encoding)
+        public StreamWriter(Stream stream, Encoding? encoding)
             : this(stream, encoding, DefaultBufferSize, false)
         {
         }
@@ -86,7 +86,7 @@ public StreamWriter(Stream stream, Encoding encoding)
         // character encoding is set by encoding and the buffer size,
         // in number of 16-bit characters, is set by bufferSize.
         //
-        public StreamWriter(Stream stream, Encoding encoding, int bufferSize)
+        public StreamWriter(Stream stream, Encoding? encoding, int bufferSize)
             : this(stream, encoding, bufferSize, false)
         {
         }
@@ -140,13 +140,13 @@ public StreamWriter(string path, bool append)
         {
         }
 
-        public StreamWriter(string path, bool append, Encoding encoding)
+        public StreamWriter(string path, bool append, Encoding? encoding)
             : this(path, append, encoding, DefaultBufferSize)
         {
         }
 
-        public StreamWriter(string path, bool append, Encoding encoding, int bufferSize) :
-            this(ValidateArgsAndOpenPath(path, append, encoding, bufferSize), encoding, bufferSize, leaveOpen: false)
+        public StreamWriter(string path, bool append, Encoding? encoding, int bufferSize) :
+            this(ValidateArgsAndOpenPath(path, append, bufferSize), encoding, bufferSize, leaveOpen: false)
         {
         }
 
@@ -155,8 +155,8 @@ public StreamWriter(string path, FileStreamOptions options)
         {
         }
 
-        public StreamWriter(string path, Encoding encoding, FileStreamOptions options)
-            : this(ValidateArgsAndOpenPath(path, encoding, options), encoding, DefaultFileStreamBufferSize)
+        public StreamWriter(string path, Encoding? encoding, FileStreamOptions options)
+            : this(ValidateArgsAndOpenPath(path, options), encoding, DefaultFileStreamBufferSize)
         {
         }
 
@@ -169,10 +169,9 @@ private StreamWriter()
             _charBuffer = Array.Empty<char>();
         }
 
-        private static FileStream ValidateArgsAndOpenPath(string path, Encoding encoding, FileStreamOptions options)
+        private static FileStream ValidateArgsAndOpenPath(string path, FileStreamOptions options)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
-            ArgumentNullException.ThrowIfNull(encoding);
             ArgumentNullException.ThrowIfNull(options);
             if ((options.Access & FileAccess.Write) == 0)
             {
@@ -182,10 +181,9 @@ private static FileStream ValidateArgsAndOpenPath(string path, Encoding encoding
             return new FileStream(path, options);
         }
 
-        private static FileStream ValidateArgsAndOpenPath(string path, bool append, Encoding encoding, int bufferSize)
+        private static FileStream ValidateArgsAndOpenPath(string path, bool append, int bufferSize)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
-            ArgumentNullException.ThrowIfNull(encoding);
             ArgumentOutOfRangeException.ThrowIfNegativeOrZero(bufferSize);
 
             return new FileStream(path, append ? FileMode.Append : FileMode.Create, FileAccess.Write, FileShare.Read, DefaultFileStreamBufferSize);
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index c043fb81ab51e..1d7f9cd468111 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -10750,15 +10750,15 @@ public partial class StreamWriter : System.IO.TextWriter
     {
         public static readonly new System.IO.StreamWriter Null;
         public StreamWriter(System.IO.Stream stream) { }
-        public StreamWriter(System.IO.Stream stream, System.Text.Encoding encoding) { }
-        public StreamWriter(System.IO.Stream stream, System.Text.Encoding encoding, int bufferSize) { }
+        public StreamWriter(System.IO.Stream stream, System.Text.Encoding? encoding) { }
+        public StreamWriter(System.IO.Stream stream, System.Text.Encoding? encoding, int bufferSize) { }
         public StreamWriter(System.IO.Stream stream, System.Text.Encoding? encoding = null, int bufferSize = -1, bool leaveOpen = false) { }
         public StreamWriter(string path) { }
         public StreamWriter(string path, bool append) { }
-        public StreamWriter(string path, bool append, System.Text.Encoding encoding) { }
-        public StreamWriter(string path, bool append, System.Text.Encoding encoding, int bufferSize) { }
+        public StreamWriter(string path, bool append, System.Text.Encoding? encoding) { }
+        public StreamWriter(string path, bool append, System.Text.Encoding? encoding, int bufferSize) { }
         public StreamWriter(string path, System.IO.FileStreamOptions options) { }
-        public StreamWriter(string path, System.Text.Encoding encoding, System.IO.FileStreamOptions options) { }
+        public StreamWriter(string path, System.Text.Encoding? encoding, System.IO.FileStreamOptions options) { }
         public virtual bool AutoFlush { get { throw null; } set { } }
         public virtual System.IO.Stream BaseStream { get { throw null; } }
         public override System.Text.Encoding Encoding { get { throw null; } }
diff --git a/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.CtorTests.cs b/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.CtorTests.cs
index bf768cf9ff725..bf0075139e699 100644
--- a/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.CtorTests.cs
+++ b/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.CtorTests.cs
@@ -48,7 +48,13 @@ public static void UnicodeEncoding()
             TestEncoding(System.Text.Encoding.Unicode, "This is Unicode\u00FF");
         }
 
-        private static void TestEncoding(System.Text.Encoding encoding, string testString)
+        [Fact]
+        public static void NullEncoding()
+        {
+            TestEncoding(null, "This is UTF8\u00FF");
+        }
+
+        private static void TestEncoding(System.Text.Encoding? encoding, string testString)
         {
             StreamWriter sw2;
             StreamReader sr2;
diff --git a/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.StringCtorTests.cs b/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.StringCtorTests.cs
index 6900fb973843b..ec94e8cb60637 100644
--- a/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.StringCtorTests.cs
+++ b/src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.StringCtorTests.cs
@@ -19,9 +19,6 @@ public static void NullArgs_ThrowsArgumentNullException()
             AssertExtensions.Throws<ArgumentNullException>("path", () => new StreamWriter((string)null, true));
             AssertExtensions.Throws<ArgumentNullException>("path", () => new StreamWriter((string)null, true, null));
             AssertExtensions.Throws<ArgumentNullException>("path", () => new StreamWriter((string)null, true, null, -1));
-            AssertExtensions.Throws<ArgumentNullException>("encoding", () => new StreamWriter("path", true, null));
-            AssertExtensions.Throws<ArgumentNullException>("encoding", () => new StreamWriter("path", null, null));
-            AssertExtensions.Throws<ArgumentNullException>("encoding", () => new StreamWriter("path", true, null, -1));
         }
 
         [Fact]
@@ -31,6 +28,7 @@ public static void EmptyPath_ThrowsArgumentException()
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter(""));
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", new FileStreamOptions()));
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", true));
+            AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", true, null));
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", true, Encoding.UTF8));
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", Encoding.UTF8, new FileStreamOptions()));
             AssertExtensions.Throws<ArgumentException>("path", () => new StreamWriter("", true, Encoding.UTF8, -1));