Rapper Big Sean -- Major Charges Dropped in Underage Sex Abuse Case

Rapper Big Sean has dodged a serious bullet -- TMZ has learned he will not be charged with sexually assaulting an underage girl back in August ... but he's not entirely in the clear.

23-year-old Sean -- who got a 2011 MTV VMA nomination for best new artist -- was arrested in August during a concert in Buffalo, NY ... after a 17-year-old girl told police the rapper and his friend had sexually assaulted her.

Big Sean -- real name Sean Michael Anderson -- was charged with 3rd degree sex abuse, forcible touching and 2nd degree unlawful imprisonment.

But now, TMZ has learned prosecutors agreed to drop the first two charges on the condition that Big Sean plead guilty to misdemeanor unlawful imprisonment ... which carries a $750 fine.