diff --git a/doc/api/fs.md b/doc/api/fs.md
index ae559d0ecff744..b0cba330c514bc 100644
--- a/doc/api/fs.md
+++ b/doc/api/fs.md
@@ -2019,11 +2019,17 @@ The file is created if it does not exist.
 
 * `'ax'` - Like `'a'` but fails if `path` exists.
 
+* `'as'` - Open file for appending in synchronous mode.
+The file is created if it does not exist.
+
 * `'a+'` - Open file for reading and appending.
 The file is created if it does not exist.
 
 * `'ax+'` - Like `'a+'` but fails if `path` exists.
 
+* `'as+'` - Open file for reading and appending in synchronous mode.
+The file is created if it does not exist.
+
 `mode` sets the file mode (permission and sticky bits), but only if the file was
 created. It defaults to `0o666` (readable and writable).
 
@@ -3920,11 +3926,17 @@ The file is created if it does not exist.
 
 * `'ax'` - Like `'a'` but fails if `path` exists.
 
+* `'as'` - Open file for appending in synchronous mode.
+The file is created if it does not exist.
+
 * `'a+'` - Open file for reading and appending.
 The file is created if it does not exist.
 
 * `'ax+'` - Like `'a+'` but fails if `path` exists.
 
+* `'as+'` - Open file for reading and appending in synchronous mode.
+The file is created if it does not exist.
+
 `mode` sets the file mode (permission and sticky bits), but only if the file was
 created. It defaults to `0o666` (readable and writable).
 
diff --git a/lib/internal/fs.js b/lib/internal/fs.js
index 9c5c4d8ad7936f..04844248b0b8e0 100644
--- a/lib/internal/fs.js
+++ b/lib/internal/fs.js
@@ -223,10 +223,14 @@ function stringToFlags(flags) {
     case 'a' : return O_APPEND | O_CREAT | O_WRONLY;
     case 'ax' : // Fall through.
     case 'xa' : return O_APPEND | O_CREAT | O_WRONLY | O_EXCL;
+    case 'as' : // Fall through.
+    case 'sa' : return O_APPEND | O_CREAT | O_WRONLY | O_SYNC;
 
     case 'a+' : return O_APPEND | O_CREAT | O_RDWR;
     case 'ax+': // Fall through.
     case 'xa+': return O_APPEND | O_CREAT | O_RDWR | O_EXCL;
+    case 'as+': // Fall through.
+    case 'sa+': return O_APPEND | O_CREAT | O_RDWR | O_SYNC;
   }
 
   throw new errors.TypeError('ERR_INVALID_OPT_VALUE', 'flags', flags);
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
index acf5c739a930c3..7f70885861ffd1 100644
--- a/test/parallel/test-fs-open-flags.js
+++ b/test/parallel/test-fs-open-flags.js
@@ -56,8 +56,12 @@ assert.strictEqual(stringToFlags('wx+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
 assert.strictEqual(stringToFlags('xw+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
 assert.strictEqual(stringToFlags('ax'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
 assert.strictEqual(stringToFlags('xa'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
+assert.strictEqual(stringToFlags('as'), O_APPEND | O_CREAT | O_WRONLY | O_SYNC);
+assert.strictEqual(stringToFlags('sa'), O_APPEND | O_CREAT | O_WRONLY | O_SYNC);
 assert.strictEqual(stringToFlags('ax+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
 assert.strictEqual(stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
+assert.strictEqual(stringToFlags('as+'), O_APPEND | O_CREAT | O_RDWR | O_SYNC);
+assert.strictEqual(stringToFlags('sa+'), O_APPEND | O_CREAT | O_RDWR | O_SYNC);
 
 ('+ +a +r +w rw wa war raw r++ a++ w++ x +x x+ rx rx+ wxx wax xwx xxx')
   .split(' ')