Chelsea capture Genk keeper Courtois
Genk said they had done everything to keep Courtois but did not succeed.
"We have no other choice but to accept Chelsea's offer," Genk, who won their third Belgian title in May, said on their website.
Chelsea manager Andre Villas-Boas later confirmed that Courtois was to undergo a medical with a view to completing the move.
"At this moment he is having a medical but has not yet agreed personal terms," he told the club's official website after arriving in Malaysia on the Blues' pre-season tour.