Leganes have conducted routine medicals on the Super Eagles star and will unveil him as their third signing of the summer on Wednesday, following the arrivals of Jose Arnaiz and Oscar Rodriguez.
The deal is believed to be a season-long loan.
24-year-old Omeruo will spend the season on loan, with an option to make the move permanent for €5m next summer, with Chelsea also agreeing a 20% future sale clause in the agreement.
The Nigerian defender is yet to play a competitive game for Chelsea since his arrival from Standard Liege in 2014.
Omeruo has played on loan at Dutch side, ADO Den Haag, English Championship side, Middlesbrough and Turkish clubs, Kasimpasa (twice) and Alanyasor.