The Diamond Rings rapper beat off stiff competition from the American rap superstars to take home the prize at the ceremony which recognizes Music Of Black Origin (MOBO) - held in Glasgow, Scotland.
In a triumphant night for British urban acts, RnB group JLS and pop/rap crossover band N-Dubz also took home two accolades each.
JLS won Best Newcomer and Best Song, for their debut single Beat Again, while N-Dubz scored Best UK Act and Best Album for their debut LP Uncle B.
JLS member Aston Merrygold said: We never thought we'd get two. We're having an incredible night.
"I am really emotional, I need two minutes to sit down and take stock.
"We only put the group together two years ago so to see how quickly things have got to this stage is amazing."
N-Dubz rapper Dappy added: "It's been great. We're going to get some food down us now and then go out to get some crazy, sexy women."
Other winners on the night included Beyonce Knowles, who won Best Video for her song Single Ladies and was named Best International act. She was not at the event, but she sent a message of thanks via video.
Best Reggae went to Sean Paul, while the show’s co-host Keri Hilson took home the award for Best RnB.
There was also a tribute to the late King of Pop Michael Jackson, which was introduced by his sister La Toya and featured performances from Michael’s brother Jermaine, 12-year-old singer Shaheen Jafargholi who performed at the Thriller singer’s memorial concert in July - and dance troupe Diversity, who won TV talent show Britain’s Got Talent this year.
MOBO Awards 2009 full list of winners:
Best UK Act: N-Dubz
Best Newcomer: JLS
Best Song: 'Beat Again', JLS
Best Album: 'Uncle B', N-Dubz
Best DJ: Trevor Nelson
Best Hip-Hop: Chipmunk
Best R and B: Keri Hilson
Best International: Beyonce Knowles
Best Video: 'Single Ladies', Beyonce Knowles
Best Reggae: Sean Paul
Best Jazz: Yolanda Brown
Best Gospel: Victizzle
Best African Act: Nneka