The Black Eagles have signed Omeruo on a season-long loan, the transfer fee and release clause are yet to be disclosed.
Besiktas had been in talks with Besiktas for over a month according to BBC African football specialist Oluwashina Okeleji who confirmed that talks between the two clubs were ongoing.
“Besiktas are in advanced talks with Chelsea but nothing has been agreed with the player,” Okeleji said.
The Nigerian international defender spent last season on-loan at Istanbul minnows Kasimpasa and established himself as a first team regular in one of the best performing defensive sides.
Besiktas are keen to strengthen in defence after being forced to field midfielder Necip Uysal in defence against Galatasaray in the Super Cup final yesterday evening.
The Black Eagles take on the newly promoted Alanyaspor for their opening game of the 2016-17 Turkish Super League season next weekend.