Ali's widow -- Lonnie -- is selling the Berrien Springs, Michigan farm that Ali bought in 1975, about 10 years before his Parkinson's diagnosis ... and it comes with a legit boxing ring in the gym.

The place is massive -- 81 acres -- and it features two houses, a bunch of garages, a steam room, a massage room and a full-length basketball court ... plus a deck overlooking the pond.

The coolest part? Its actual listing price is $2,895,037 ... 'cause Lonnie wanted to make sure Ali's 37 KOs in his career were included somewhere in the price tag.