The Nigerian starred on loan with the La Liga side last term, and has now made the move to Spain on a full-time basis
Leganes have acquired defender Kenneth Omeruo from Chelsea on a permanent transfer, the clubs have announced.
Omeruo, 25, starred with Leganes on loan last term, making 31 appearances for the club, 30 of which were starts.
Earlier this month, Goal reported that Chelsea were holding out for £6 million (€7m/$8m) for Omeruo, but the Blues have accepted a bid of around £4.6m.
Omeruo signed for Chelsea in 2012 from Standard Liege, but was never able to make the breakthrough into the first team at Stamford Bridge.
The Nigeria international took in several loan stints during his time at Chelsea, including with ADO Den Haag in the Netherlands and Middlesbrough in England, as well as Kasımpasa and Alanyaspor in Turkey.
But Omeruo found a home at Leganes in his final loan spell with the Blues, and parlayed his strong performances last term into a permanent move.