Spurs agreed a deal worth a guaranteed £85m for the Portuguese, with the transfer officially confirmed by the Premier League club on Thursday.
Saints are owed some of the profit after they inserted a reported 15 per cent sell-on clause when they sold Fernandes for £42m last summer.
Fernandes has signed a long-term contract in north London after attracting interest from Manchester United, Paris Saint-Germain and Real Madrid.
New Tottenham head coach Roberto De Zerbi played a decisive role in his choice of move, as Fernandes revealed in his first interview with the club.
“I’m very excited for this next step. Spurs is a massive club, and the head coach was a key part of why I have decided to join,” Fernandes said.
“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.
“I can’t wait to get started, meet the fans, and give everything for the club.”
Fernandes joined Saints from Sporting Lisbon in 2024 following a successful loan spell at Estoril and enjoyed a standout individual campaign.
Despite the club's relegation from the Premier League, the 21-year-old scored four goals and registered seven assists in 39 appearances for Saints.
Now, after just one season at London Stadium, following a £42m move after Saints were relegated, Fernandes has secured another big-money move.
De Zerbi added: “I’ve admired Mateus for a long time because he combines quality on the ball with the intensity and intelligence.
"Despite his age, he already has good experience in the Premier League and has shown quality and consistency.
“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 transfer represents another significant financial boost for Saints, who have now generated almost £50m from a player they signed for £15m.