diff --git a/Package.swift b/Package.swift
index a781bbcb8..de4a218fc 100644
--- a/Package.swift
+++ b/Package.swift
@@ -330,5 +330,9 @@ let package = Package(
             name: "SmithyHTTPAPITests",
             dependencies: ["SmithyHTTPAPI"]
         ),
+        .testTarget(
+            name: "SmithyStreamsTests",
+            dependencies: ["SmithyStreams", "Smithy"]
+        ),
     ].compactMap { $0 }
 )
diff --git a/Sources/Smithy/Stream.swift b/Sources/Smithy/Stream.swift
index 43a351812..6ab719578 100644
--- a/Sources/Smithy/Stream.swift
+++ b/Sources/Smithy/Stream.swift
@@ -64,6 +64,7 @@ public enum StreamError: Error, Sendable {
     case invalidOffset(String)
     case notSupported(String)
     case connectionReleased(String)
+    case writeToClosedStream(String)
 }
 
 extension Stream {
diff --git a/Sources/SmithyStreams/BufferedStream.swift b/Sources/SmithyStreams/BufferedStream.swift
index d720597f2..702ea2bc7 100644
--- a/Sources/SmithyStreams/BufferedStream.swift
+++ b/Sources/SmithyStreams/BufferedStream.swift
@@ -8,6 +8,7 @@
 import struct Foundation.Data
 import class Foundation.NSRecursiveLock
 import protocol Smithy.Stream
+import enum Smithy.StreamError
 
 /// A `Stream` implementation that buffers data in memory.
 /// The buffer size depends on the amount of data written and read.
@@ -207,8 +208,15 @@ public class BufferedStream: Stream, @unchecked Sendable {
     /// Writes the specified data to the stream.
     /// Then, continues a suspended reader (if any) to read the data.
     /// - Parameter data: The data to write.
+    /// - Throws: `StreamError.writeToClosedStream` if a write is attempted after the stream is closed.
     public func write(contentsOf data: Data) throws {
-        lock.withLockingClosure {
+        try lock.withLockingClosure {
+            // Do not allow writing to stream once it closes.
+            // This ensures length of stream does not change once it reports a length.
+            // Throw `StreamError.writeToClosedStream` to alert the caller.
+            guard !isClosed else {
+                throw StreamError.writeToClosedStream("Attempt to write to closed stream")
+            }
             // append the data to the buffer
             // this will increase the in-memory size of the buffer
             _buffer.append(data)
diff --git a/Tests/ClientRuntimeTests/NetworkingTests/Streaming/BufferedStreamTests.swift b/Tests/SmithyStreamsTests/BufferedStreamTests.swift
similarity index 89%
rename from Tests/ClientRuntimeTests/NetworkingTests/Streaming/BufferedStreamTests.swift
rename to Tests/SmithyStreamsTests/BufferedStreamTests.swift
index 052e13ef0..2c08699a5 100644
--- a/Tests/ClientRuntimeTests/NetworkingTests/Streaming/BufferedStreamTests.swift
+++ b/Tests/SmithyStreamsTests/BufferedStreamTests.swift
@@ -6,7 +6,7 @@
 //
 
 import XCTest
-import ClientRuntime
+import enum Smithy.StreamError
 import class SmithyStreams.BufferedStream
 
 final class BufferedStreamTests: XCTestCase {
@@ -208,6 +208,29 @@ final class BufferedStreamTests: XCTestCase {
         XCTAssertEqual(testData + additionalData, readData1)
     }
 
+    func test_write_throwsWriteToClosedStreamErrorWhenStreamIsClosed() throws {
+        let subject = BufferedStream(data: testData, isClosed: true)
+        XCTAssertThrowsError(
+            try subject.write(contentsOf: additionalData),
+            "No error was thrown when writing to a closed stream",
+            { error in
+                switch error {
+                case StreamError.writeToClosedStream:
+                    break // expected error, test passes
+                default:
+                    XCTFail("Error was thrown, but not of expected error type")
+                }
+            }
+        )
+    }
+
+    func test_write_rejectsAdditionalDataWhenStreamIsClosed() throws {
+        let subject = BufferedStream(data: testData, isClosed: true)
+        XCTAssertThrowsError(try subject.write(contentsOf: additionalData))
+        let readData = try subject.read(upToCount: Int.max)
+        XCTAssertEqual(testData, readData)
+    }
+
     // MARK: - length
 
     func test_length_returnsNilLengthBeforeStreamCloses() throws {