Uncle Sam to MC Hammer -- Pay Me My $700,000 in Taxes!

The U.S. government isn't letting MC Hammer off the hook for the $779,585 it claims the rapper owes in back taxes -- and has now filed a lawsuit against the rapper and his wife ... to recoup its losses.

According to the docs, obtained by TMZ, the back taxes are from Hammer's most financially troubled years -- 1996 and 1997 -- and Uncle Sam wants the money back stat, demanding any concert earnings Hammer makes be automatically redirected into the government's pocket.

Hammer -- real name Stanley Burrell -- filed for bankruptcy in 1996, after burying himself in $13 million of debt.