The Premier League club announced the signing on Tuesday evening with Ugochukwu, 19, signing a seven-year deal.
He made his senior Rennes debut in 2021 and went on to make 60 appearances for his boyhood club, scoring one Ligue 1 goal in the process.
Ugochukwu follows Nicolas Jackson, Christopher Nkunku and Angelo Gabriel to west London, with defender Axel Disasi’s move from Monaco expected to be completed in the coming days.