diff --git a/CHANGELOG.md b/CHANGELOG.md
index 47145a41d4..42420eae55 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 
 - Add `nativeStackAndroid` support to `NativeLinkedErrors`, capturing the JVM stack trace of rejected native module promises as a linked exception ([#6278](https://github.com/getsentry/sentry-react-native/pull/6278))
 - Record XHR request/response headers and (optionally) bodies in Mobile Session Replay. Opt in via `mobileReplayIntegration` with `networkDetailAllowUrls` to capture headers; set `networkCaptureBodies: true` to also capture bodies. Other options: `networkDetailDenyUrls`, `networkRequestHeaders`, `networkResponseHeaders`. Authorization-like headers are always stripped, bodies are capped at ~150 KB. Covers XHR-based clients like `axios`; fetch will follow. See [Network Details](https://docs.sentry.io/platforms/react-native/session-replay/#network-details) for details. ([#6288](https://github.com/getsentry/sentry-react-native/pull/6288))
+- Use the optional `react-native-quick-base64` peer dependency for envelope base64 encoding when installed, to improve `captureEnvelope` performance. Falls back to the bundled JS encoder if the package is absent. ([#6314](https://github.com/getsentry/sentry-react-native/pull/6314))
 - Warn during dev builds when multiple versions of Sentry JS SDK are detected ([#6269](https://github.com/getsentry/sentry-react-native/pull/6269))
 
 ### Fixes
diff --git a/packages/core/package.json b/packages/core/package.json
index 34626fe6b4..4ad5f9bfac 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -70,7 +70,8 @@
   "peerDependencies": {
     "expo": ">=49.0.0",
     "react": ">=17.0.0",
-    "react-native": ">=0.65.0"
+    "react-native": ">=0.65.0",
+    "react-native-quick-base64": ">=3.0.0"
   },
   "dependencies": {
     "@sentry/babel-plugin-component-annotate": "5.3.0",
@@ -130,6 +131,9 @@
   "peerDependenciesMeta": {
     "expo": {
       "optional": true
+    },
+    "react-native-quick-base64": {
+      "optional": true
     }
   }
 }
diff --git a/packages/core/src/js/utils/base64.ts b/packages/core/src/js/utils/base64.ts
new file mode 100644
index 0000000000..e4c55456bf
--- /dev/null
+++ b/packages/core/src/js/utils/base64.ts
@@ -0,0 +1,56 @@
+import { debug } from '@sentry/core';
+
+import { base64StringFromByteArray } from '../vendor';
+
+type FromByteArray = (bytes: Uint8Array, urlSafe?: boolean) => string;
+
+let cachedEncoder: FromByteArray | null = null;
+let resolved = false;
+
+/**
+ * Resolves the base64 encoder once. If the optional peer dependency
+ * `react-native-quick-base64` is installed, its native JSI encoder is used
+ * (~10x faster than the pure-JS fallback). Otherwise the bundled JS encoder
+ * from `vendor/base64-js` is used.
+ *
+ * The resolution is cached so the require cost is paid at most once.
+ */
+function resolveEncoder(): FromByteArray {
+  if (resolved) {
+    return cachedEncoder ?? base64StringFromByteArray;
+  }
+  resolved = true;
+
+  try {
+    // Optional peer dependency — only loaded if the consumer installed it.
+    // eslint-disable-next-line @typescript-eslint/no-var-requires, import/no-extraneous-dependencies
+    const quickBase64 = require('react-native-quick-base64') as { fromByteArray?: FromByteArray };
+    if (quickBase64 && typeof quickBase64.fromByteArray === 'function') {
+      cachedEncoder = quickBase64.fromByteArray;
+      debug.log('Using react-native-quick-base64 for envelope encoding.');
+      return cachedEncoder;
+    }
+  } catch (_e) {
+    // Not installed — fall through to JS encoder.
+  }
+
+  cachedEncoder = base64StringFromByteArray;
+  return cachedEncoder;
+}
+
+/**
+ * Encode a byte array to a base64 string. Prefers the native
+ * `react-native-quick-base64` encoder when available, otherwise uses the
+ * bundled JS implementation.
+ */
+export function encodeToBase64(input: Uint8Array): string {
+  return resolveEncoder()(input);
+}
+
+/**
+ * @internal Test helper. Resets the cached encoder so the next call re-resolves.
+ */
+export function _resetBase64EncoderForTesting(): void {
+  cachedEncoder = null;
+  resolved = false;
+}
diff --git a/packages/core/src/js/wrapper.ts b/packages/core/src/js/wrapper.ts
index d4225740d6..f53cfa5726 100644
--- a/packages/core/src/js/wrapper.ts
+++ b/packages/core/src/js/wrapper.ts
@@ -30,11 +30,11 @@ import type { MobileReplayOptions } from './replay/mobilereplay';
 import type { RequiredKeysUser } from './user';
 
 import { isHardCrash } from './misc';
+import { encodeToBase64 } from './utils/base64';
 import { encodeUTF8 } from './utils/encode';
 import { isTurboModuleEnabled } from './utils/environment';
 import { convertToNormalizedObject } from './utils/normalize';
 import { ReactNativeLibraries } from './utils/rnlibraries';
-import { base64StringFromByteArray } from './vendor';
 import { SDK_VERSION } from './version';
 
 /**
@@ -231,7 +231,7 @@ export const NATIVE: SentryNativeWrapper = {
       envelopeBytes = newBytes;
     }
 
-    await RNSentry.captureEnvelope(base64StringFromByteArray(envelopeBytes), { hardCrashed });
+    await RNSentry.captureEnvelope(encodeToBase64(envelopeBytes), { hardCrashed });
   },
 
   /**
diff --git a/packages/core/test/utils/base64.test.ts b/packages/core/test/utils/base64.test.ts
new file mode 100644
index 0000000000..9305f546aa
--- /dev/null
+++ b/packages/core/test/utils/base64.test.ts
@@ -0,0 +1,43 @@
+import { _resetBase64EncoderForTesting, encodeToBase64 } from '../../src/js/utils/base64';
+
+jest.mock(
+  'react-native-quick-base64',
+  () => ({
+    __esModule: true,
+    fromByteArray: jest.fn((bytes: Uint8Array) => `quick:${bytes.length}`),
+  }),
+  { virtual: true },
+);
+
+describe('encodeToBase64', () => {
+  beforeEach(() => {
+    _resetBase64EncoderForTesting();
+    jest.resetModules();
+  });
+
+  test('uses react-native-quick-base64 when available', () => {
+    // The module mock above provides a fake `fromByteArray` returning a sentinel.
+    const result = encodeToBase64(new Uint8Array([0x73, 0x65, 0x6e, 0x74, 0x72, 0x79]));
+    expect(result).toBe('quick:6');
+  });
+
+  test('falls back to the JS encoder when react-native-quick-base64 is not installed', () => {
+    jest.isolateModules(() => {
+      jest.doMock(
+        'react-native-quick-base64',
+        () => {
+          throw new Error('Cannot find module');
+        },
+        { virtual: true },
+      );
+      // eslint-disable-next-line @typescript-eslint/no-var-requires
+      const {
+        encodeToBase64: isolatedEncode,
+        _resetBase64EncoderForTesting: isolatedReset,
+      } = require('../../src/js/utils/base64');
+      isolatedReset();
+      // "sentry" => "c2VudHJ5"
+      expect(isolatedEncode(new Uint8Array([0x73, 0x65, 0x6e, 0x74, 0x72, 0x79]))).toBe('c2VudHJ5');
+    });
+  });
+});
diff --git a/yarn.lock b/yarn.lock
index a5beec674d..9d57f24b17 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -10931,9 +10931,12 @@ __metadata:
     expo: ">=49.0.0"
     react: ">=17.0.0"
     react-native: ">=0.65.0"
+    react-native-quick-base64: ">=3.0.0"
   peerDependenciesMeta:
     expo:
       optional: true
+    react-native-quick-base64:
+      optional: true
   bin:
     sentry-eas-build-on-complete: scripts/eas-build-hook.js
     sentry-eas-build-on-error: scripts/eas-build-hook.js