Boro boss Aitor Karanka confirmed after Saturday's FA Cup loss to Hull City that he was closing in on Omeruo, and now he has finalised a deal.
The 20-year-old, who can play at centre-back and full-back, is yet to make his debut for Chelsea but he has 14 caps for Nigeria.
"Kenneth is a strong defender and I think we need to improve in this area," said Karanka.
"I know him Jose (Mourinho) and has told me about him and I think he's going to be very good for us to add to the squad."
Omeruo is now set to go straight into Boro's squad for this weekend's clash at Blackpool.