The striker leaves Elland Road after three years and his two previous clubs, Oxford United and West Brom, will both benefit financially from sell-on clauses.
Leeds will have to share 15 per cent of the profit they’ve made on the frontman, who cost £3m in 2016.
Roofe made 122 appearances for the Whites and scored 32 goals, including 15 last season which made him top goalscorer for Marcelo Bielsa’s side.
The Whites did make a contract offer, but when it wasn’t accepted and with his current deal due to run out next summer, the club opted to cash in.
It is understood Anderlecht made the player a lucrative offer to prise him away from English football.
A club statement wished him well as he joins Vincent Kompany’s side, saying: “We thank Kemar for his efforts whilst at the club and wish him all the best for the future.”