West Ham have signed midfielder Mateus Fernandes from Southampton in a deal worth more than £40m.

The 21-year-old Portuguese joins on a five-year contract and becomes West Ham's third-biggest signing, behind Sebastien Haller (£45m) and Lucas Paqueta (£51m).

Fernandes moved to Southampton from Sporting for £15m in 2024, so the Saints have made a substantial profit after one year.

He made 46 appearances for Southampton, scoring four goals and providing seven assists.

"I'm very excited to play for West Ham," said Fernandes.

"I think it's a big step for me. It's a big club, a massive club. The project, the stadium, the city, everything."

The move should be a boost for under-pressure Hammers manager Graham Potter, whose side have lost their first three matches of the season in all competitions.

They were beaten 3-0 by newly-promoted Sunderland in their first Premier League outing before being thrashed 5-1 by Chelsea, and lost to Wolves in the second round of the Carabao Cup.

West Ham are also close to completing the signing of Monaco midfielder Soungoutou Magassa, 21, for a fee of about £17.3m.