diff --git a/CHANGELOG.md b/CHANGELOG.md
index ca8b55a4f..e769dc497 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Removed
 
 ### Fixed
+- Fix mutability of connection headers ([#291](https://github.com/opensearch-project/opensearch-js/issues/291))
 
 ### Security
 
diff --git a/lib/Connection.js b/lib/Connection.js
index 2e4a5a547..bd71d2c53 100644
--- a/lib/Connection.js
+++ b/lib/Connection.js
@@ -217,7 +217,7 @@ class Connection {
       origin: url.origin,
       // https://github.com/elastic/elasticsearch-js/issues/843
       port: url.port !== '' ? url.port : undefined,
-      headers: this.headers,
+      headers: Object.assign({}, this.headers),
       agent: this.agent,
     };
 
diff --git a/test/unit/connection.test.js b/test/unit/connection.test.js
index cb0458dc9..d9f0268db 100644
--- a/test/unit/connection.test.js
+++ b/test/unit/connection.test.js
@@ -625,6 +625,37 @@ test('Custom headers for connection', (t) => {
   });
 });
 
+test('mutability of connection headers', (t) => {
+  t.plan(3);
+
+  function handler(req, res) {
+    t.match(req.headers, {
+      'x-foo': 'bar',
+    });
+    req.headers['x-custom-test'] = true;
+    res.end('ok');
+  }
+
+  buildServer(handler, ({ port }, server) => {
+    const connection = new Connection({
+      url: new URL(`http://localhost:${port}`),
+      headers: { 'x-foo': 'bar' },
+    });
+    connection.request(
+      {
+        path: '/hello',
+        method: 'GET',
+      },
+      (err) => {
+        t.error(err);
+        // should not update the default
+        t.same(connection.headers, { 'x-foo': 'bar' });
+        server.stop();
+      }
+    );
+  });
+});
+
 // TODO: add a check that the response is not decompressed
 test('asStream set to true', (t) => {
   t.plan(2);