Just days after the UFC superstar announced his retirement "from the sport formerly known as 'Mixed Martial Art'" ... The Notorious was in a boxing ring -- and TMZ Sports has the vid.

You can see clear as day McGregor is still in great shape and still very ready to kick some ass ... sparring with a guy at Fight Club Gym on Wednesday in Miami and getting in some ab work.

Based on the footage, seems obvious McGregor is staying ready just in case he decides to unretire and get back to work in the UFC.

As we previously reported, McGregor has talked passionately recently about wanting rematches with both Khabib Nurmagomedov and Nate Diaz.

We recently spoke with UFC honcho Dana White who thinks Conor is serious about retirement -- since he's made so money already and seems to be successful in his Irish whiskey endeavor.

But, watch the clip ... does this look like a guy who's done fighting to you?