The Portuguese youngster moves to London stadium on a five-year contract and for an undisclosed fee.
“I'm very happy to be here. 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.
“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.”
Hammers boss Graham Potter also stated: “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 with Southampton and is someone we really feel can help improve us and fit into what we are trying to build and develop at the Club.
“We believe that his attributes and strengths will bring a different dimension to the squad and, from speaking to him, I know that he can’t wait to get started here.
“He has shown great maturity and character for someone so young, and we look forward to seeing his continued progress and impact at West Ham United.”