diff --git a/src/js/internal/fs/streams.ts b/src/js/internal/fs/streams.ts
index 939ce1153ea..bfce25ce7eb 100644
--- a/src/js/internal/fs/streams.ts
+++ b/src/js/internal/fs/streams.ts
@@ -560,6 +560,9 @@ function _write(data, encoding, cb) {
   const fileSink = this[kWriteStreamFastPath];
 
   if (fileSink && fileSink !== true) {
+    if (typeof data === "string" && encoding && encoding !== "utf8" && encoding !== "utf-8" && encoding !== "buffer") {
+      data = Buffer.from(data, encoding);
+    }
     const maybePromise = fileSink.write(data);
     if ($isPromise(maybePromise)) {
       maybePromise
@@ -603,6 +606,9 @@ function underscoreWriteFast(this: FSStream, data: any, encoding: any, cb: any)
       this.fd = fileSink._getFd();
     }
 
+    if (typeof data === "string" && encoding && encoding !== "utf8" && encoding !== "utf-8" && encoding !== "buffer") {
+      data = Buffer.from(data, encoding);
+    }
     const maybePromise = fileSink.write(data);
     if ($isPromise(maybePromise)) {
       maybePromise.then(
@@ -652,6 +658,9 @@ function writeFast(this: FSStream, data: any, encoding: any, cb: any) {
 
   const fileSink = this[kWriteStreamFastPath];
   if (fileSink && fileSink !== true) {
+    if (typeof data === "string" && encoding && encoding !== "utf8" && encoding !== "utf-8" && encoding !== "buffer") {
+      data = Buffer.from(data, encoding);
+    }
     const maybePromise = fileSink.write(data);
     if ($isPromise(maybePromise)) {
       maybePromise
diff --git a/test/js/node/process/process-stdio.test.ts b/test/js/node/process/process-stdio.test.ts
index ba35bc4f048..c271d5687de 100644
--- a/test/js/node/process/process-stdio.test.ts
+++ b/test/js/node/process/process-stdio.test.ts
@@ -158,4 +158,48 @@ describe.concurrent("process-stdio", () => {
       `hello worldhello again|😋 Get Emoji — All Emojis to ✂️ Copy and 📋 Paste 👌`.repeat(9999),
     );
   });
+
+  // Regression: process.stdout.write(string, encoding) was ignoring the encoding
+  // parameter on the fast path and UTF-8 encoding the string instead.
+  test.each(["binary", "latin1"] as const)("process.stdout.write(string, '%s') writes raw bytes", encoding => {
+    const { stdout } = spawnSync({
+      cmd: [
+        bunExe(),
+        "-e",
+        `for (let i = 0; i <= 0xff; i++) process.stdout.write(String.fromCharCode(i), ${JSON.stringify(encoding)});`,
+      ],
+      stdout: "pipe",
+      stdin: null,
+      stderr: "inherit",
+      env: bunEnv,
+    });
+    expect(stdout).toBeInstanceOf(Buffer);
+    expect(stdout.length).toBe(256);
+    const expected = Buffer.alloc(256);
+    for (let i = 0; i < 256; i++) expected[i] = i;
+    expect(Buffer.compare(stdout, expected)).toBe(0);
+  });
+
+  test("process.stdout.write(string, 'hex') decodes hex", () => {
+    const { stdout } = spawnSync({
+      cmd: [bunExe(), "-e", `process.stdout.write("deadbeef", "hex");`],
+      stdout: "pipe",
+      stdin: null,
+      stderr: "inherit",
+      env: bunEnv,
+    });
+    expect(Buffer.compare(stdout, Buffer.from([0xde, 0xad, 0xbe, 0xef]))).toBe(0);
+  });
+
+  test("process.stdout.write(string) defaults to UTF-8", () => {
+    const { stdout } = spawnSync({
+      cmd: [bunExe(), "-e", `process.stdout.write("héllo");`],
+      stdout: "pipe",
+      stdin: null,
+      stderr: "inherit",
+      env: bunEnv,
+    });
+    // é = U+00E9 = UTF-8 c3 a9
+    expect(Buffer.compare(stdout, Buffer.from([0x68, 0xc3, 0xa9, 0x6c, 0x6c, 0x6f]))).toBe(0);
+  });
 });