The 22-year-old star - who was a judge on the US version of the show for two years from 2012 - is said to have agreed to join the UK version after it was claimed Simon Cowell has sacked Mel B after just one series.
A source told Daily Mirror newspaper: ''Simon's management phoned to see when Demi could sign her contract about a week ago.
''They both have busy schedules but she is planning to do it as soon as possible and Simon is eager to get the judging panel sorted.''