She added: "I feel so good, it feels better than I imagined."
She said she hoped her album, on which she raps and sings, and which relies heavily on jazz and soul as well as hip-hop, had thrown a "real wrench" into the music business.
"There's a lot of music that sounds the same, all day on the radio," she told journalists.
"Hopefully people will hear this album and realise they don't have to make music that sounds the same - they can make music that sounds good."
She is the prize's first female winner since 2002, when Ms Dynamite picked up the trophy.
Speech Debelle: Better Days
"Ms Dynamite was an inspiration for me," she said. "Seeing her win the award made it a reality for me. She looks like me, she talks like me, and made me think, oh, maybe I can do that."
The winner was picked from a shortlist of 12 albums by a panel of judges.
Music critic Jude Rogers, one of the panel members, said Debelle had made a "wonderful record".
"It does present a real freshness and individuality," she said.
"The thing I love about this record is that it is a rap record which doesn't sound like a rap record.
"It brings in many influences from jazz, folk, blues, even old TV soundtrack music. It's very British, which I think is wonderful.
"It has a joy and vitality and passion about it that it really does share with the previous winners."
Speech Therapy was released on the Big Dada record label, which is also home to rapper Roots Manuva, who contributed to the album.
RECENT MERCURY WINNERS
2008: Elbow - The Seldom Seen Kid
2007: Klaxons - Myths of the Near Future
2006: Arctic Monkeys - Whatever People Say I Am, That's What I'm Not
2005: Antony and the Johnsons - I Am a Bird Now
2004: Franz Ferdinand - Franz Ferdinand
2003: Dizzee Rascal - Boy in da Corner
2002: Ms Dynamite - A Little Deeper
Reviewers have picked up on the contrast between her "honeyed" delivery and the gritty subject matter of the songs on the album which includes living in hostels and absent fathers and gangsters.
Before the win, it had sold fewer than 3,000 copies - too few to reach the top 40.
Martin Talbot, of the Official Charts Company said: "It is one of the least established albums in the history of the Mercurys and an album which still has great potential.
"The Mercury recognition will play a big part in helping achieve the broader public recognition which it deserves."
Florence and the Machine had been the 5/2 favourite with bookmakers for her number two album Lungs.
Other acts fancied by bookmakers at the start of the night included The Horrors with Primary Colours and singer-songwriter Bat For Lashes - whose nomination for Two Suns marks her second time as a Mercury contender.
Bat For Lashes performed Moon and Moon from album Two Suns
Lisa Hannigan, Led Bib, Sweet Billy Pilgrim and The Invisible were also nominated.
The 12 albums were shortlisted based solely on musical merit without reference to record sales or live performances, organisers said.
Albums released between July 2008 and July 2009 and made by British or Irish artists were eligible for the prize.
Previous winners of the prize have included Elbow, Klaxons, Arctic Monkeys, Antony and the Johnsons, and Dizzee Rascal.
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