Play video content EXCLUSIVE TMZ.com

Tekashi69 climbing to the top of the charts doesn't make him immune to the harsh reality -- if ya do the crime, ya gotta do the time ... so says Kevin O'Leary.

We got the "Shark Tank" star outside Benjamin Prime Steak House in NYC -- Tekashi's home turf -- and he surprised us in a couple of ways. For starters, the 64-year-old entrepreneur knows exactly who 6ix9ine is. And, get this ... Kevin's a fan.

Check it out ... Kevin -- an expert in brand-building -- says Tekashi's monumentally successful album, "Dummy Boy," means he won't have to worry about money ... even while he's sitting in a federal prison facility.