The Nigeria international moves to the Championship club on-loan for the remainder of the season. There is no option to buy.
Sevilla announced on Monday evening: "Sevilla and Middlesbrough have reached an agreement for the loan, until the end of this season, of Nigerian international striker Kelechi Iheanacho.
"The Sevilla striker, who arrived at the club last summer after his contract with Leicester City expired, has played eleven games for the red and whites, scoring three goals. In this way, Iheanacho returns to English football, where he will strengthen the Riverside Stadium team in its aim of returning to the Premier League.
"The Sevilla club would like to thank Iheanacho for his work in recent months and wishes him the best of luck for this second part of the season."