The Belgian champions said on their website that the 20-year-old would stay with them on loan until June.
The bustling De Bruyne, who has played twice for Belgium, is Genk's joint second highest scorer this season with 11 goals. He played twice against Chelsea in the Champions League group stage earlier this season.
Belgian media said Chelsea had paid 8 million euros. Chelsea were expected to officially comment on the deal later on Tuesday, the final day of the January transfer window.
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