TMZ Sports has learned Rob Gronkowski just picked up a BEAUTIFUL new pad in Downtown Miami for $1.7 million ... and he bought the place from another famous jock!!

As we previously reported, the former (?) New England Patriots tight end recently sold his Boston bachelor pad after announcing he was retiring from the NFL.

And, it seems he's moving into enemy AFC East territory ... scooping up a 3,850 square foot condo in the famous Marquis Miami building on Biscayne Blvd.

The place is SICK!!!!! Amazing beach views, super high ceilings (he's very tall) with 5 bedrooms and 4.5 bathrooms ... perfect for those wild Gronk parties!

It also comes with 3 parking spaces ... and if you know Downtown Miami, you know those are hard to come by!!

The building is top notch ... valet, pool, fitness center and spa -- and it's close to nightclubs!

And, it's great for Gronk's girlfriend, Camille Kostek, too -- since Miami is the epicenter of the modeling world in the U.S. ... and Camille is the Sports Illustrated Swimsuit Edition cover model!

We're told Gronk, represented by Sebastian Wagner of Douglas Elliman, bought the place from retired Norwegian soccer player John Carew, who was represented by John Sandberg and Ann Nortmann of Douglas Elliman -- so there's an athlete-to-athlete connection!

We reached out to all of the agents involved -- but no comment from anyone.