The Hammers, which have lost their first two top-flight games and were dumped out of the Carabao Cup by Wolverhampton Wanderers in midweek, will part with more than £40 million ($82.5 million) for the 21-year-old.

Fernandes becomes the third-most expensive player in West Ham's history, after creative midfielder Lucas Paqueta and striker Sebastien Haller.

He only joined Southampton from Sporting CP in a £15 million ($31 million) deal last year, before impressing in 2024-2025 despite the Saints being relegated to the EFL Championship with a mere 12 points.

Fernandes played 36 Premier League games last term, 34 of them starts, and provided two goals and four assists.

His 34 chances created were the joint-most by a Southampton player alongside Ryan Manning, and he has also appeared in all three of its Championship games under Will Still this campaign.

Having penned a five-year contract with West Ham, Fernandes said: "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. 

"I want to try to play football, enjoy football, not just run and try to score, but try to play good football.  I think the most important thing is to try to give everything every day."

Fernandes's arrival comes as a major boost to boss Graham Potter, who is already under pressure ahead of this a trip to Nottingham Forest.

He is West Ham's sixth signing of the transfer window, after the arrivals of Callum Wilson, Kyle Walker-Peters, Mads Hermansen and El Hadji Malick Diouf, while Jean-Clair Todibo's loan move was made permanent for £35 million ($72.2million).