Is React Native's Async Storage secure?

AsyncStorage is not suitable for storing sensitive information. You might find this useful: https://github.com/oblador/react-native-keychain

It uses facebook conceal/android keystore to store encrypted data to SharedPreferences (Android) and keychain on iOS. (I co-authored the lib). Be sure to read the entire readme to understand what it offers.

No , it is not secure since it is not encrypted .I would recommend that you use Expo`s secureStore

If you`re building your app from Expo :

// in managed apps:
import { SecureStore } from 'expo';

If you`re building as a bare app

// in bare apps:
import * as SecureStore from 'expo-secure-store';

Read more here : https://docs.expo.io/versions/v32.0.0/sdk/securestore/