The duo had a helluva weekend together in Miami ... clubbing, playing softball and eating expensive ass meals out in South Beach.

But, while the good times may fade away, there's one part of their bro-down that won't -- the wild new ink O'Malley got ... with Tekashi taking the matters into his own hands and doing the job himself!!

You can see shirtless O'Malley sitting while Tekashi, needle in hand ... is going to work on his right shoulder.

Granted, celebrity tattoo artist Tatu Panda was there to make sure Tekashi69 didn't screw up ... and eventually took over after the first few minutes.

O'Malley was pleased with his big and bold 69 tat ... saying "you da man!" in the comment section on the Instagram post.

But, Suga wasn't the only one to get tatted by Tekashi ... it looks like YouTuber SteveWillDoIt let him start a tat on his left hand, too!!

No word on if 6ix9ine got a tat as well ... but the dude's clearly running out of room on his body, so who knows?!

Suga recapped the whole weekend on his podcast ... saying Steve even gifted him a sick new Rolex in the process!!