The Hammers will pay an initial 38 million pound fee, with an additional 4 million pounds in performance-related add-ons. The 21-year-old has signed a five-year contract and becomes the club's third-most expensive signing, behind Lucas Paqueta (51 million pounds) and Sebastien Haller (45 million pounds).
Fernandes joined Southampton from Sporting CP in August 2024 for 15 million pounds. After an impressive debut season in England, where he made a total of 46 appearances across all competitions - scoring four goals and providing seven assists - the Saints have turned a significant profit just one year later.
Speaking to West Ham's official media, Fernandes expressed his excitement.
"I'm very happy to be here. I'm very excited to play for West Ham. I think it's a big step for me. It's a big club, a massive club. The project, the stadium, the city - everything attracted me. I want to try to play football, enjoy football, not just run and try to score, but try to play good football. The most important thing is to give everything every day."
In the Premier League last season, Fernandes featured 36 times for Southampton, notching two goals and four assists, while gaining vital top-flight experience.
West Ham manager Graham Potter praised the acquisition, highlighting Fernandes' potential and maturity.
"Mateus is a young player who has made impressive progress in the early part of his career, and we are delighted to have secured his signing. He gained valuable experience in the Premier League last year and is someone we really feel can help improve us and fit into what we are trying to build here," said Potter.
"He brings a different dimension to the squad and has shown great maturity and character for someone so young. We look forward to seeing his continued progress and impact."
Fernandes' arrival comes at a crucial time for Potter, with West Ham under pressure following a poor start to the season. The Hammers have lost their opening three matches in all competitions, including a 3-0 defeat to newly-promoted Sunderland, a 5-1 thrashing by Chelsea, and an early Carabao Cup exit to Wolves.
In addition to Fernandes, West Ham are reportedly close to finalizing another midfield signing, with 21-year-old Soungoutou Magassa from AS Monaco expected to join for around 17.3 million pounds.
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