Muhammad Ali -- MY FIRST SELFIE ... Is the Greatest!

He's only been on Instagram for 3 weeks, but Muhammad Ali has already posted his very first selfie.

The 72-year-old champ was decked out in Louisville shirt to cheer on the Cardinals as they took on the Kentucky Wildcats this weekend.

Ali wrote, "#Louisville Game Day! Go Cards!"

It's the first time Ali has posted a new photo on any social media platform since he joined Twitter in 2010.