Rap Legend Slick Rick Becomes a U.S. Citizen!

Slick Rick threw on his brand new Gucci underwear, went down to the court and became a U.S. citizen after 40 years in the country.

We got shots of Rick celebrating outside a NYC courthouse where he was sworn in on Friday morning. The British born rap legend said, "I am so proud of this moment -- and so honored to finally become an American citizen."

Rick the Ruler's had immigration issues since 1990 when he did prison time for a self-defense shooting. He was eventually pardoned, but was threatened with deportation several times.

He's been working on becoming a citizen for more than 23 years. Rick says he'll have dual citizenship ... he's not giving up his British passport.

Rick moved to the Bronx in 1976 with his parents, and eventually met Doug E. Fresh in school. The rest is hip-hop history.