Onuoha has agreed a season-long loan with the Black Cats.
“Nedum is a modern-day defender,” Sunderland boss Steve Bruce told the club’s official website.
“He’s big, quick and powerful – a really good defender, who, despite his age, has already played over 100 games. He has all the attributes a modern defender needs.
“Nedum is someone I’ve tracked for a long time so it’s great to get the deal done to bring him here. He can play anywhere across the back line, too, so it’s great to have added that flexibility and some strong competition for places.”