Boxing legend Leon Spinks -- who famously beat Muhammad Ali in 1978 -- is fighting for his life in a Vegas hospital, TMZ Sports has learned ... and his wife is asking for prayers.

Details surrounding the 66-year-old's condition are unclear -- but we know his family and friends are very concerned.

In fact, Leon's wife, Brenda, went to social media saying, "Reaching out to ask you to kindly send some prayers out for my Beautiful Husband Leon so that he may overcome the obstacles that have crossed his path."

Spinks has fought back from serious medical situations in the past -- he was hospitalized in 2014 after a piece of bone from a chicken wing caused major damage to his intestines.

The boxer underwent several surgeries and spent weeks recovering at a rehabilitation center.

Spinks is a boxing legend -- he was inducted into the Nevada Boxing Hall of Fame in 2017.

His 1978 victory over Muhammad Ali to win the heavyweight championship of the world is widely considered one of the greatest upsets in boxing history.

Spinks is also a military veteran who served in the U.S. Marines from 1973 to 1976.