diff --git a/src/benchmarks/micro/libraries/System.IO/BinaryWriterExtendedTests.cs b/src/benchmarks/micro/libraries/System.IO/BinaryWriterExtendedTests.cs
index bf64a67cd7b..dae03a5b81c 100644
--- a/src/benchmarks/micro/libraries/System.IO/BinaryWriterExtendedTests.cs
+++ b/src/benchmarks/micro/libraries/System.IO/BinaryWriterExtendedTests.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using BenchmarkDotNet.Attributes;
 using MicroBenchmarks;
 
@@ -10,8 +11,10 @@ namespace System.IO.Tests
     [BenchmarkCategory(Categories.Libraries)]
     public class BinaryWriterExtendedTests
     {
-        private string _input;
-        private char[] _inputAsChars;
+        private string _asciiInput;
+        private char[] _asciiInputAsChars;
+        private string _nonAsciiInput;
+        private char[] _nonAsciiInputAsChars;
         private BinaryWriter _bw;
 
         [Params(32, 8_000, 2_000_000)]
@@ -22,22 +25,96 @@ public void Setup()
         {
             _bw = new BinaryWriter(new NullWriteStream());
 
-            _input = new string('x', StringLengthInChars);
-            _inputAsChars = _input.ToCharArray();
+            _asciiInput = new string('x', StringLengthInChars);
+            _asciiInputAsChars = _asciiInput.ToCharArray();
+            // '\u00E0' (à) encodes to 2 bytes in UTF-8, exercising multi-byte encoding paths
+            _nonAsciiInput = new string('\u00E0', StringLengthInChars);
+            _nonAsciiInputAsChars = _nonAsciiInput.ToCharArray();
         }
 
         [Benchmark]
         [MemoryRandomization]
         public void WriteAsciiCharArray()
         {
-            _bw.Write(_inputAsChars);
+            _bw.Write(_asciiInputAsChars);
         }
 
         [Benchmark]
         [MemoryRandomization]
         public void WriteAsciiString()
         {
-            _bw.Write(_input);
+            _bw.Write(_asciiInput);
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteNonAsciiCharArray()
+        {
+            _bw.Write(_nonAsciiInputAsChars);
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteNonAsciiString()
+        {
+            _bw.Write(_nonAsciiInput);
+        }
+    }
+
+    /// <summary>
+    /// Benchmarks for BinaryWriter with a non-UTF-8 encoding, exercising the
+    /// _useFastUtf8 = false code path.
+    /// </summary>
+    [BenchmarkCategory(Categories.Libraries)]
+    public class BinaryWriterUnicodeEncodingTests
+    {
+        private string _asciiInput;
+        private char[] _asciiInputAsChars;
+        private string _nonAsciiInput;
+        private char[] _nonAsciiInputAsChars;
+        private BinaryWriter _bw;
+
+        [Params(32, 8_000, 2_000_000)]
+        public int StringLengthInChars;
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            _bw = new BinaryWriter(new NullWriteStream(), Encoding.Unicode);
+
+            _asciiInput = new string('x', StringLengthInChars);
+            _asciiInputAsChars = _asciiInput.ToCharArray();
+            // '\u00E0' (à) encodes to 2 bytes in UTF-8, exercising multi-byte encoding paths
+            _nonAsciiInput = new string('\u00E0', StringLengthInChars);
+            _nonAsciiInputAsChars = _nonAsciiInput.ToCharArray();
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteAsciiCharArray()
+        {
+            _bw.Write(_asciiInputAsChars);
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteAsciiString()
+        {
+            _bw.Write(_asciiInput);
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteNonAsciiCharArray()
+        {
+            _bw.Write(_nonAsciiInputAsChars);
+        }
+
+        [Benchmark]
+        [MemoryRandomization]
+        public void WriteNonAsciiString()
+        {
+            _bw.Write(_nonAsciiInput);
         }
     }
 }
diff --git a/src/benchmarks/micro/libraries/System.IO/BinaryWriterTests.cs b/src/benchmarks/micro/libraries/System.IO/BinaryWriterTests.cs
index 7a9f9e5293f..55e67e014db 100644
--- a/src/benchmarks/micro/libraries/System.IO/BinaryWriterTests.cs
+++ b/src/benchmarks/micro/libraries/System.IO/BinaryWriterTests.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using BenchmarkDotNet.Attributes;
 using MicroBenchmarks;
 
@@ -78,4 +79,33 @@ public void WriteDouble()
             _bw.Write((double)Math.PI);
         }
     }
+
+    /// <summary>
+    /// Benchmarks for BinaryWriter with a non-UTF-8 encoding, exercising the
+    /// _useFastUtf8 = false code path for Write(char).
+    /// </summary>
+    [BenchmarkCategory(Categories.Libraries)]
+    public class BinaryWriterUnicodeEncodingCharTests
+    {
+        private BinaryWriter _bw;
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            _bw = new BinaryWriter(new NullWriteStream(), Encoding.Unicode);
+        }
+
+        [Benchmark]
+        public void WriteAsciiChar()
+        {
+            _bw.Write('a');
+        }
+
+        [Benchmark]
+        public void WriteNonAsciiChar()
+        {
+            // '\u00E0' (à) encodes to 2 bytes in UTF-8, exercising multi-byte encoding paths
+            _bw.Write('\u00E0');
+        }
+    }
 }