Omeruo, 24, moved to Chelsea from Standard Liege in 2012, but is yet to make a first-team appearance for the club.
The defender has previously represented Middlesbrough on loan, but spent last season with Turkish outfit Kasimpasa.
Omeruo, who started twice for Nigeria at the 2018 World Cup, has now left Stamford Bridge once again, with Leganes securing his services for their La Liga campaign.
"Kenneth Omeruo will spend the 2018-19 season on loan at Leganes in the Spanish top flight," confirmed a statement on Chelsea's official website.
Chelsea currently have more than 30 players out on loan.