The 20-year-old has signed a five-and-a-half-year deal and will remain at the Belgian champions on loan until the end of the season.
De Bruyne, who is predominantly a left-sided player, has been on Chelsea's radar for some time and has played twice against the London club in this year's Champions League group stages.
A statement on Genk's website read: "The transfer of Kevin De Bruyne to Chelsea has been completed. He has signed for five and a half seasons and will continue (playing with) Genk until June.
"His signature was confirmed at 0.30 last night. De Bruyne and his agent will return back today from London, along with general director Dirk Degraen.
"Genk will hold a press conference with (director) Jacob Gunther and Dirk Degraen in which they will clarify the transfer policy.
"Afterwards, Kevin De Bruyne will also give an explanation about his transfer to the English club."