It's understood Spurs have paid a club record fee rising to £100m for the midfielder.
The 26-year-old is a familiar face in north London, having first faced Spurs while at Milan in the Champions League back in 2023. Since moving to Newcastle United, he played a key role in the club's first trophy since 1969, last year's League Cup triumph, and their return to the Champions League. Capped 32 times by Italy, he now swaps black and white for Lilywhite, having worn the number 16 shirt.
Tonali said the decision to join Spurs had felt instant once talks began: "I'm very happy to be here. When I arrived at the Club today, it felt fantastic. People said about there being four or five clubs - there was only one. I spoke to the Head Coach for close to two hours about the Club, the fans, the stadium and our football.
"It was like magic because I knew immediately that I had to sign for Tottenham. I've played against Tottenham a few times and always found a great atmosphere made by great fans. I can't wait to start the season."
He described the emotions of his first morning at the club in similar terms.
"This morning at 6am I woke up as a Tottenham player, and when we came here this morning the feeling was fantastic, it was like when I was 16, 17 and I played my first game. It was so similar. I was like a kid with the energy inside. I think this is all positive, especially because I'm 26 now, not 18 years old, I decide to sign for Tottenham. I don't need two teams, three teams, four teams - I know already my team for the next season and I can't wait to start the season and to play the first game."
Tonali also spoke about the challenge of settling into a new dressing room, pointing to an existing relationship with his new head coach as a help.
"Every time you change team you have to restart everything – your relationship with team-mates, with the staff, really with the fans. I'm a bit lucky because when you know the staff, it's more easy for you, especially at the start of the season, the summer."
Sporting director Johan Lange said Tonali's arrival strengthened the club's midfield options significantly, "Sandro is one of the best midfielders in Europe and we are delighted to welcome him to the Club. He has outstanding technical quality to go with real football intelligence, and has the character to thrive in a demanding, high-pressure environment.
"Sandro brings valuable experience at the highest level, both domestically and in European competition and I know our supporters will love his energy and commitment on the pitch. We are all excited to see him pull on our famous Lilywhite shirt for the first time."
Spurs manager Roberto De Zerbi, who shares a hometown connection with the midfielder, welcomed the signing personally: "Sandro is a special player and a great signing for our Club. I have followed him for a long time, as he came through the youth system at my hometown club, Brescia, and I'm so happy to be working with him now. Given his qualities, there was a lot of interest in Sandro this summer. However, he was very clear in his desire to join Tottenham, and I know our fans will love what he brings to the team."