diff --git a/src/lib/index.ts b/src/lib/index.ts
index 3fc1630..201213a 100644
--- a/src/lib/index.ts
+++ b/src/lib/index.ts
@@ -2,7 +2,7 @@ import ArrayIo, { ArrayOptions } from './type/ArrayIo.js';
 import StringIo, { StringOptions } from './type/StringIo.js';
 import StructIo, { StructFields, StructOptions } from './type/StructIo.js';
 import TlvIo, { TlvValueCallback } from './type/TlvIo.js';
-import { validateType } from './util.js';
+import { IoMode, validateType } from './util.js';
 import { openStream } from './stream/util.js';
 import { IoContext, IoSource, IoStream, IoType } from './types.js';
 
@@ -162,6 +162,7 @@ const float64le = {
 
 export {
   IoContext,
+  IoMode,
   IoSource,
   IoStream,
   IoType,
diff --git a/src/lib/stream/FsStream.ts b/src/lib/stream/FsStream.ts
index 370ae79..68b73d9 100644
--- a/src/lib/stream/FsStream.ts
+++ b/src/lib/stream/FsStream.ts
@@ -1,5 +1,5 @@
 import fs from '../shim/fs.cjs';
-import { Endianness } from '../util.js';
+import { Endianness, IoMode } from '../util.js';
 import { IoStream } from '../types.js';
 
 class FsStream implements IoStream {
@@ -12,13 +12,17 @@ class FsStream implements IoStream {
   #littleEndian: boolean;
   #offset: number;
 
-  constructor(source: string | number, endianness = Endianness.Little) {
+  constructor(
+    source: string | number,
+    mode = IoMode.Read,
+    endianness = Endianness.Little,
+  ) {
     if (fs === undefined) {
       throw new Error('fs api unavailable');
     }
 
     if (typeof source === 'string') {
-      this.#fd = fs.openSync(source, 'r');
+      this.#fd = fs.openSync(source, mode === IoMode.Write ? 'w+' : 'r');
       this.#ownFd = true;
     } else if (typeof source === 'number') {
       this.#fd = source;
diff --git a/src/lib/stream/util.ts b/src/lib/stream/util.ts
index 337e6c0..e27b3fc 100644
--- a/src/lib/stream/util.ts
+++ b/src/lib/stream/util.ts
@@ -1,6 +1,6 @@
 import ArrayBufferStream from './ArrayBufferStream.js';
 import FsStream from './FsStream.js';
-import { Endianness } from '../util.js';
+import { Endianness, IoMode } from '../util.js';
 import { IoSource, IoStream } from '../types.js';
 
 const isStream = (ref: any) => {
@@ -15,12 +15,13 @@ const isStream = (ref: any) => {
 
 const openStream = (
   source: IoSource,
+  mode = IoMode.Read,
   endianness = Endianness.Little,
 ): IoStream => {
   if (isStream(source)) {
     return source as IoStream;
   } else if (typeof source === 'string' || typeof source === 'number') {
-    return new FsStream(source, endianness);
+    return new FsStream(source, mode, endianness);
   } else if (ArrayBuffer.isView(source)) {
     return new ArrayBufferStream(
       source.buffer,
diff --git a/src/lib/type/ArrayIo.ts b/src/lib/type/ArrayIo.ts
index 72c272e..76cb852 100644
--- a/src/lib/type/ArrayIo.ts
+++ b/src/lib/type/ArrayIo.ts
@@ -1,4 +1,4 @@
-import { validateType } from '../util.js';
+import { IoMode, validateType } from '../util.js';
 import { openStream } from '../stream/util.js';
 import { IoContext, IoSource, IoType } from '../types.js';
 
@@ -32,7 +32,7 @@ class ArrayIo implements IoType {
   }
 
   read(source: IoSource, context: IoContext = {}) {
-    const stream = openStream(source);
+    const stream = openStream(source, IoMode.Read);
     const value = [];
     const size = this.#options.size;
 
@@ -53,7 +53,7 @@ class ArrayIo implements IoType {
   }
 
   write(source: IoSource, value: any[], context: IoContext = {}) {
-    const stream = openStream(source);
+    const stream = openStream(source, IoMode.Write);
     const size = value.length;
     const type = this.#type;
 
diff --git a/src/lib/type/StringIo.ts b/src/lib/type/StringIo.ts
index 57cbe1f..7b7f431 100644
--- a/src/lib/type/StringIo.ts
+++ b/src/lib/type/StringIo.ts
@@ -1,4 +1,4 @@
-import { resolveValue } from '../util.js';
+import { IoMode, resolveValue } from '../util.js';
 import { openStream } from '../stream/util.js';
 import { IoContext, IoSource, IoStream, IoType } from '../types.js';
 
@@ -49,7 +49,7 @@ class StringIo implements IoType {
   }
 
   read(source: IoSource, context: IoContext = {}) {
-    const stream = openStream(source);
+    const stream = openStream(source, IoMode.Read);
     const size = resolveValue(this.#options.size, context.local, context.root);
 
     const rawBytes = this.#readRawBytes(stream, size);
@@ -59,7 +59,7 @@ class StringIo implements IoType {
   }
 
   write(source: IoSource, value: string, context: IoContext = {}) {
-    const stream = openStream(source);
+    const stream = openStream(source, IoMode.Write);
 
     const finalValue = this.#options.reverse
       ? value.split('').reverse().join('')
diff --git a/src/lib/type/StructIo.ts b/src/lib/type/StructIo.ts
index f0ff5ad..9f89cb1 100644
--- a/src/lib/type/StructIo.ts
+++ b/src/lib/type/StructIo.ts
@@ -1,4 +1,4 @@
-import { Endianness, validateType } from '../util.js';
+import { Endianness, IoMode, validateType } from '../util.js';
 import { openStream } from '../stream/util.js';
 import { IoContext, IoSource, IoType } from '../types.js';
 
@@ -44,7 +44,7 @@ class StructIo implements IoType {
   }
 
   read(source: IoSource, context: IoContext = {}) {
-    const stream = openStream(source, this.#options.endianness);
+    const stream = openStream(source, IoMode.Read, this.#options.endianness);
     const value: Record<string, any> = {};
 
     context.local = value;
@@ -58,7 +58,7 @@ class StructIo implements IoType {
   }
 
   write(source: IoSource, value: object, context: IoContext = {}) {
-    const stream = openStream(source, this.#options.endianness);
+    const stream = openStream(source, IoMode.Write, this.#options.endianness);
 
     context.local = value;
     context.root = context.root ?? value;
diff --git a/src/lib/type/TlvIo.ts b/src/lib/type/TlvIo.ts
index a628ecc..e4e7b00 100644
--- a/src/lib/type/TlvIo.ts
+++ b/src/lib/type/TlvIo.ts
@@ -1,4 +1,4 @@
-import { Endianness } from '../util.js';
+import { Endianness, IoMode } from '../util.js';
 import { openStream } from '../stream/util.js';
 import { IoContext, IoSource, IoType } from '../types.js';
 
@@ -58,7 +58,7 @@ class TlvIo implements IoType {
   }
 
   read(source: IoSource, context: IoContext = {}): Tlv {
-    const stream = openStream(source, this.#options.endianness);
+    const stream = openStream(source, IoMode.Read, this.#options.endianness);
 
     context.local = null;
     context.root = context.root ?? null;
@@ -71,7 +71,11 @@ class TlvIo implements IoType {
 
     let valueValue = valueBytes;
     if (valueType && valueType.read) {
-      const valueStream = openStream(valueBytes, this.#options.endianness);
+      const valueStream = openStream(
+        valueBytes,
+        IoMode.Read,
+        this.#options.endianness,
+      );
       valueValue = valueType.read(valueStream, context);
     }
 
@@ -83,7 +87,7 @@ class TlvIo implements IoType {
   }
 
   write(source: IoSource, value: Tlv, context: IoContext = {}) {
-    const stream = openStream(source, this.#options.endianness);
+    const stream = openStream(source, IoMode.Write, this.#options.endianness);
 
     context.local = null;
     context.root = context.root ?? null;
diff --git a/src/lib/util.ts b/src/lib/util.ts
index 396db7a..4c3ad26 100644
--- a/src/lib/util.ts
+++ b/src/lib/util.ts
@@ -5,6 +5,11 @@ enum Endianness {
   Big = 2,
 }
 
+enum IoMode {
+  Read = 1,
+  Write = 2,
+}
+
 const validateType = (type: IoType) => {
   if (typeof type.getSize !== 'function') {
     throw new Error('Missing required function: getSize');
@@ -28,4 +33,4 @@ const resolveValue = (ref: number | string, ...objects: object[]) => {
   }
 };
 
-export { Endianness, validateType, resolveValue };
+export { Endianness, IoMode, validateType, resolveValue };
diff --git a/src/spec/stream/util.spec.ts b/src/spec/stream/util.spec.ts
index 57917e2..f322e7f 100644
--- a/src/spec/stream/util.spec.ts
+++ b/src/spec/stream/util.spec.ts
@@ -2,6 +2,8 @@ import { describe, expect, test } from 'vitest';
 import FsStream from '../../lib/stream/FsStream.js';
 import ArrayBufferStream from '../../lib/stream/ArrayBufferStream.js';
 import { openStream } from '../../lib/stream/util.js';
+import { IoMode } from '../../lib/util.js';
+import * as fs from 'fs';
 
 describe('openStream', () => {
   test('returns FsStream object when given path to file', () => {
@@ -9,6 +11,13 @@ describe('openStream', () => {
     expect(stream).toBeInstanceOf(FsStream);
   });
 
+  test('returns FsStream object when given path to nonexistent file in write mode', () => {
+    const stream = openStream('./fixture/nonexistent.file', IoMode.Write);
+    expect(stream).toBeInstanceOf(FsStream);
+    stream.close();
+    fs.rmSync('./fixture/nonexistent.file');
+  });
+
   test('returns ArrayBufferStream object when given Uint8Array', () => {
     const view = new Uint8Array(10);
     const stream = openStream(view);