The 20-year-old joined Chelsea last year from French side Stade Rennes for around 30 million euros and has since made 15 appearances for the west London club.
A product of the Rennes Academy, Ugochukwu made his debut in 2021 and played 60 games in total for their first team.
"Southampton have a great history," the France under-21 international said in a statement.
"A lot of great players came here and played for Southampton, so I'm very happy to be part of the team now. It was a very easy decision."
Southampton have also signed forward Cameron Archer from Aston Villa on a four-year contract, reportedly worth £15 million.
They visit Newcastle United in their Premier League opener on Saturday.