The midfielder initially joined the Sky Blues in January and has now made the move permanent after the opening of the summer transfer window.
Onyeka played a key role in Frank Lampard’s side winning the Sky Bet Championship title, making 14 appearances and scoring a crucial goal in a victory over Derby County in April.
The 28-year-old brings valuable experience from the Premier League, having made 75 top-flight appearances during his time with Brentford.
“I’m really excited for the new season with Coventry,” Onyeka told club media.
“When I first joined last season, I came here with two goals and that was to win the Championship and help Coventry get promoted to the Premier League.
“I’m really happy to now be a Coventry player back in the Premier League.
“I will give everything to the team, fight for every ball, fight for my teammates and bring my experience.
“My ambition now is to help the Club stay in the Premier League and finish as high as we can.”