diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 106830f75682..bb861e5e22d5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -87,6 +87,8 @@ jobs:
           - latest
         os:
           - ubuntu-latest
+          - windows-latest
+          - macOS-latest
 
     steps:
       - name: Clone repository
diff --git a/fs/_get_fs_flag.ts b/fs/_get_fs_flag.ts
index 2fbabeb7cbf7..8b53f9288cf3 100644
--- a/fs/_get_fs_flag.ts
+++ b/fs/_get_fs_flag.ts
@@ -1,6 +1,6 @@
 // Copyright 2018-2025 the Deno authors. MIT license.
 
-import { getNodeFs } from "./_utils.ts";
+import { getNodeFs, getNodeOs } from "./_utils.ts";
 import type { WriteFileOptions } from "./unstable_types.ts";
 import type { OpenOptions } from "./unstable_open.ts";
 
@@ -22,11 +22,23 @@ type OpenBooleanOptions = Pick<
 export function getWriteFsFlag(opt: WriteBooleanOptions): number {
   const { O_APPEND, O_CREAT, O_EXCL, O_TRUNC, O_WRONLY } =
     getNodeFs().constants;
-
-  let flag = O_WRONLY;
-  if (opt.create) {
+  const { platform } = getNodeOs();
+
+  // On Windows: The O_CREAT flag is set by default to prevent throwing an
+  // EINVAL error (code -4071) when running Node on a Windows OS. The O_CREAT
+  // flag will create a new file if the file does not exist and is a no-op when
+  // the file exists on Windows. This makes the `WriteBooleanOption`,
+  // `{ create: true }`, the default option. Passing `{ create: false }` will
+  // throw an Error.
+  let flag = platform() !== "win32" ? O_WRONLY : O_CREAT | O_WRONLY;
+
+  if (platform() === "win32" && !opt.create) {
+    flag ^= O_CREAT;
+  }
+  if (platform() !== "win32" && opt.create) {
     flag |= O_CREAT;
   }
+
   if (opt.createNew) {
     flag |= O_EXCL;
   }
diff --git a/fs/unstable_write_file_test.ts b/fs/unstable_write_file_test.ts
index cdf9836e8c89..34259b678575 100644
--- a/fs/unstable_write_file_test.ts
+++ b/fs/unstable_write_file_test.ts
@@ -83,9 +83,16 @@ Deno.test("writeFile() handles 'create' when writing to a file", async () => {
   const data = encoder.encode("Hello");
 
   // Rejects with NotFound when file does not initally exist.
-  await assertRejects(async () => {
-    await writeFile(testFile, data, { create: false });
-  }, NotFound);
+  // TODO: This should throw NotFound on Windows.
+  if (platform() === "win32") {
+    await assertRejects(async () => {
+      await writeFile(testFile, data, { create: false });
+    }, Error);
+  } else {
+    await assertRejects(async () => {
+      await writeFile(testFile, data, { create: false });
+    }, NotFound);
+  }
 
   // Creates a file that does not initially exist. (This is default behavior).
   await writeFile(testFile, data, { create: true });
@@ -96,7 +103,11 @@ Deno.test("writeFile() handles 'create' when writing to a file", async () => {
 
   // Overwrites the existing file with new content.
   const dataAgain = encoder.encode("Hello, Standard Library");
-  await writeFile(testFile, dataAgain, { create: false });
+  if (platform() === "win32") {
+    await writeFile(testFile, dataAgain);
+  } else {
+    await writeFile(testFile, dataAgain, { create: false });
+  }
   const dataReadAgain = await readFile(testFile);
   const readDataAgain = decoder.decode(dataReadAgain);
   assertEquals(readDataAgain, "Hello, Standard Library");
@@ -332,9 +343,16 @@ Deno.test("writeFileSync() handles 'create' when writing to a file", () => {
   const data = encoder.encode("Hello");
 
   // Throws with NotFound when file does not initally exist.
-  assertThrows(() => {
-    writeFileSync(testFile, data, { create: false });
-  }, NotFound);
+  // TODO: This should throw NotFound on Windows.
+  if (platform() === "win32") {
+    assertThrows(() => {
+      writeFileSync(testFile, data, { create: false });
+    }, Error);
+  } else {
+    assertThrows(() => {
+      writeFileSync(testFile, data, { create: false });
+    }, NotFound);
+  }
 
   // Creates a file that does not initially exist. (This is default behavior).
   writeFileSync(testFile, data, { create: true });
@@ -345,7 +363,11 @@ Deno.test("writeFileSync() handles 'create' when writing to a file", () => {
 
   // Overwrites the existing file with new content.
   const dataAgain = encoder.encode("Hello, Standard Library");
-  writeFileSync(testFile, dataAgain, { create: false });
+  if (platform() === "win32") {
+    writeFileSync(testFile, dataAgain);
+  } else {
+    writeFileSync(testFile, dataAgain, { create: false });
+  }
   const dataReadAgain = readFileSync(testFile);
   const readDataAgain = decoder.decode(dataReadAgain);
   assertEquals(readDataAgain, "Hello, Standard Library");
diff --git a/fs/unstable_write_text_file_test.ts b/fs/unstable_write_text_file_test.ts
index 874c300d7566..3e2215321198 100644
--- a/fs/unstable_write_text_file_test.ts
+++ b/fs/unstable_write_text_file_test.ts
@@ -93,9 +93,16 @@ Deno.test("writeTextFile() handles 'create' for a file", async () => {
   const tempDirPath = await makeTempDir({ prefix: "writeTextFile_" });
   const testFile = join(tempDirPath, "testFile.txt");
 
-  await assertRejects(async () => {
-    await writeTextFile(testFile, "Hello", { create: false });
-  }, NotFound);
+  // TODO: This should throw NotFound on Windows.
+  if (platform() === "win32") {
+    await assertRejects(async () => {
+      await writeTextFile(testFile, "Hello", { create: false });
+    }, Error);
+  } else {
+    await assertRejects(async () => {
+      await writeTextFile(testFile, "Hello", { create: false });
+    }, NotFound);
+  }
 
   await writeTextFile(testFile, "Hello", { create: true });
   const readData = await readTextFile(testFile);
@@ -300,9 +307,16 @@ Deno.test("writeTextFileSync() handles 'create' for a file", () => {
   const tempDirPath = makeTempDirSync({ prefix: "writeTextFileSync_" });
   const testFile = join(tempDirPath, "testFile.txt");
 
-  assertThrows(() => {
-    writeTextFileSync(testFile, "Hello", { create: false });
-  }, NotFound);
+  // TODO: This should throw NotFound on Windows.
+  if (platform() === "win32") {
+    assertThrows(() => {
+      writeTextFileSync(testFile, "Hello", { create: false });
+    }, Error);
+  } else {
+    assertThrows(() => {
+      writeTextFileSync(testFile, "Hello", { create: false });
+    }, NotFound);
+  }
 
   writeTextFileSync(testFile, "Hello", { create: true });
   const readData = readTextFileSync(testFile);