Rapper Fat Joe -- who's made MILLIONS cranking out hits like "What's Luv" and "Make It Rain" -- just plead guilty to tax evasion ... after failing to pay Uncle Sam a FORTUNE in back taxes.

The 42-year-old rapper -- real name Joseph Cartagena -- was accused of stiffing the federal government out of roughly $700k in income taxes for 2007 and 2008.

Joe -- who appeared in federal court in New Jersey this morning to enter the plea -- was freed on a $250k bond. He's scheduled to be sentenced in April and could get up to two years in prison.