The 20-year-old spent the second half of last season at the Riverside Stadium, making 14 appearances for the Championship club.
Omeruo now hopes that he will be able to gain more first-team experience during a second spell with Boro.
"I decided to return to Boro because I really enjoyed my last loan there and working with the manager Aitor Karanka," he is quoted as saying by BBC Sport.
"I like the atmosphere at Middlesbrough, the players and fans too are very supportive. It's such an easy choice to make and a key factor in going there is the chance to play first-team football."
Omeruo started every game for Nigeria during the recent World Cup in Brazil.