The mum-of-three, 37, will join auditions for the talent show in Los Angeles this month, a source confirmed to MailOnline.
Mel has long been a favourite of show boss Simon Cowell, and was a popular stand-in judge on the X Factor UK last year.
Got talent: Mel B as she reunites with the Spice Girls at the closing ceremony of the London 2012 Olympic Games. She's now joining AGT.
Reunited: The Spice Girls at the closing ceremony at the London 2012 Olympics - on the back of London black cabs
The show - which is the world’s biggest selling TV format - moved to New York from Los Angeles especially to accommodate radio star Stern, 58, who replaced Piers Morgan.
Sharon Osbourne, 60, announced her departure from AGT last July – and then declared she had left after the way NBC executives had treated her son Jack following his diagnosis with multiple sclerosis.
She told the New York Post that Jack had been axed from an upcoming NBC show called Stars Earn Stripes, saying: ‘I just can’t be fake. It’s discrimination, and it was badly handled.’
A source told MailOnline: 'Simon loves trying people out on his shows, Mel's already been a judge on X Factor Australia and it's too far for her to keep taking the kids.
'She did want to do X Factor UK, but Simon really wanted her to do America's Got Talent.
'She's outspoken and never afraid to speak her mind, she makes for good TV. NBC execs will be watching through their eyes - let's hope they won't be using the delay button on live TV.'
Mel's got the X Factor: Mel B with X Factor UK judges, Gary Barlow, Tulisa Contostavlos and Louis Walsh.
Farewell Mrs O: Mel B will replace Sharon Osbourne on AGT after the feisty star quit following a row with NBC chiefs
Mrs and Mr Nasty: Mel B at the auditions for the UK X Factor last year - and show boss Simon Cowell
She said: 'I wanted to fall asleep through that, I'm sorry I have to be honest. It's a no.'
However, her fellow judge Tulisa Contostavlos didn't see anything wrong with the singer's no-nonsense approach to judging and believes her comments were only what the majority of people were thinking.
The singer said: 'I can't say anything bad about her. I just thought she was saying what everyone else was thinking. Mel B just keeps it real.'
Proud mum: Mel B takes her girls to Mr Chow's in LA to celebrate daughter Phoenix's 14th birthday yesterday
Together again: The Spice Girls - minus Victoria Beckham - at the premiere of their musical, Viva Forever, in London in December
She swiftly told one group: 'The dance routine was lazy and sloppy and one of you was doing it half-a****.
Mel, who is married to Stephen Belafonte, is mum to daughter Phoenix, who celebrated her 14th birthday yesterday, as well as Angel, five, and one-year-old Madison.
Last summer, she reunited with Victoria Beckham, Mel C, Geri Halliwell and Emma Bunton to perform at the closing ceremony of the London 2012 Olympics - and the former band performed on the back of London cabs.
Following their performance, it was reported the band would reunite for an album and a 'final fling' tour - however, Victoria refused as she wanted to concentrate on her successful fashion brand.
It had been rumoured that Cowell would put his sometime paramour Carmen Electra in Sharon's seat - that that report was wide off the mark.
During September’s season finale, America's Got Talent had 11.86 million viewers.
However, 2011's AGT climax drew nearly 15.5 million viewers.