ESP-01S 1MB WIFI ESP8266 WiFi Serial Transceiver Wireless Module

You could add WiFi capability to your Arduino project by using this module.

This module integrates with the ESP8266 IC. It has integrated TCP/IP protocol stack. It also promotes a complete WiFi networking solution, making it possible to host an application or taking all WiFi networking functions from the different application processors.

ESP8266 also has a powerful on-board processing and storage capability, meaning you could use this module alone in creating a simple project.

You could integrate this with your MCU by connecting it using the UART(RX/TX) and the power supply (3.3V/GND) to make this work.