The attacking midfielder, who is part of the youth set-up at Genk, caught United’s attention when he played in a friendly against them in April 2013.
Boonen has been training with United for the last month and he says he has now put pen to paper on a professional deal after turning 16, turning down reported interest from Manchester City, Arsenal and Feyenoord among others.
“It’s like in a dream,” he is quoted as saying by Belgian newspaper HLN. “The training sessions are hard, but I don’t feel the tiredness when I realise I have signed for??Manchester United.”
United have also reportedly been tracking the progress of Boonen’s 13-year-old brother Seppe, who is a goalkeeper, although he would not be able to sign for anyone until he turns 16.
Manchester United are yet to confirm the deal.