The 21-year-old joins the North London side for a reported £85million fee as they look to revamp the squad after narrowly avoiding Premier League relegation in the last campaign.
"I'm very excited for this next step," said Fernandes to the club website.
"Spurs is a massive club and the Head Coach [Roberto De Zerbi] was a key part of why I have decided to join. When we spoke, it was very special. We look at football in the same way - going onto the pitch as a strong team, with fight and energy, to try and win every game.
"I can't wait to get started, to meet the fans, to meet everyone, and give everything for the Club."
Fernandes made 38 appearances for the Hammers in all competitions, scoring four goals while also providing four assists, earning himself his first international cap for Portugal as a late substitute in a friendly against the United States in April.
Meanwhile, new Tottenham manager De Zerbi also heaped praise on his latest signing, revealing his long-time admiration of the player.
"I've admired Mateus for a long time because he combines quality on the ball with the intensity and intelligence that are so important in the way we want to play," said the Italian.
"Despite his age, he already has good experience in the Premier League and has shown quality and consistency at this level. Mateus is comfortable under pressure, can progress the ball, works hard for the team and has the courage to make things happen in difficult moments.
"I believe this is the ideal environment for him to continue his development and I'm excited to start working with him."
The youngster will wear shirt number 18 for Spurs.
Manchester City have completed the signing of England midfielder Elliot Anderson from Nottingham Forest in a transfer worth 116 million pounds, making the 23-year-old the most expensive British player in football history.
Portugal head coach Roberto Martinez said his side were ready to begin what he described as a "second World Cup" as they prepared to face Croatia in the Round of 32 in Toronto on Thursday.
Senegal striker Nicolas Jackson is set to return to Chelsea after Bayern Munich decided against signing him after last season's loan spell in German Bundesliga.
Nottingham Forest are expected to announce Oliver Glasner as the new manager ahead of the upcoming 2026/27 season after the two parties agreed terms.
