J.J. Watt -- Mystery Cabin Revealed ... AND IT'S FREAKIN' SICK

Just because J.J. Watt bought a cabin in the middle of nowhere, doesn't mean he's gotta rough it -- 'cause TMZ Sports has the first pic of J.J.'s new Wisconsin pad ... and it's AWESOME!

Watt bought the digs last month as a private getaway for secluded off-season training purposes, describing the home as "really minimalistic" and a way to totally commit to his regimen.

Well, when you have a $100 million contract "minimalistic" runs around $800,000 and features 4,500 square feet of living space ... with 4 bedrooms, 2.5 bathrooms and oh yeah ... an elevator.

The property is just outside Milwaukee -- and sits on 35 acres of midwestern wilderness, perfect for "Rocky IV" style Arctic level training.