The 31-year-old will be joining Bengaluru from UCAM Murcia, a Segunda B (Spanish third division) side, for the 2019/20 season.
The Spaniard has plied his trade for Osasuna, Dinamo Tbilisi, Lleida Esportiu, Lorca and Murcia, finishing as the top goalscorer of the 2018/19 season for the latter.
Onwu won the 2015 Georgian Cup with Dinamo Tbilisi and helped Lorca to promotion to the Segunda division.
The striker will be the fourth Spaniard in the current Bengaluru team as he joins the likes of Dimas Delgado, Albert Serran and Juanan Gonzalez.
The 31-year-old will learn that he has huge boots to fill as he looks to replace the outgoing prolific Venezuelan forward Miku. His experience of three seasons in the top flight of Spanish football will help Bengaluru in their quest to defend the title.