The 20-year-old made 14 appearances for Boro last term after joining the club on loan in January, and Head Coach Aitor Karanka was delighted to welcome him back.
"He is a very important player to us," said Aitor. "He's someone I love to have with us because he's a player who came here from Chelsea and his mood was always perfect.
"He was always happy and fighting for his position.
"He was handed the opportunity to play in the World Cup and he is now coming back to us a better player."
Omeruo will add further strength to the squad ahead of the season opener against Birmingham City.