It marks the first signing of the post-Daniel Levy era.
The England midfielder joins Spurs from Atletico Madrid in a deal worth in the region of £34.7 million, returning to London on a long-term contract until summer 2031, according to the Press Association.
Gallagher said: "I’m so happy and excited to be here, taking the next step in my career at an amazing club.
"I wanted to be a Spurs player and thankfully the club felt the same.
"It was very easy, it happened very quickly and I’m ready to get on the pitch.
"I know how great the fans are, I’m really happy to be a part of it here and want to create special moments and memories together."
Gallagher, who came through the Chelsea academy and scored 10 goals in 95 appearances for Chelsea, had been on Tottenham's radar for some time but opted to join Atletico in 2024.
He made 77 appearances during 18 months in Spain but struggled to secure a regular starting spot under manager Diego Simeone.
The 25-year-old will be hoping that a return to the Premier League helps him regain the attention of England boss Thomas Tuchel, who last selected him for the national squad in June.
His arrival comes at a time when Tottenham are short of midfield options.
Rodrigo Bentancur recently underwent hamstring surgery, Lucas Bergvall remains sidelined, and Pape Sarr is away on international duty at the Africa Cup of Nations.
Spurs head coach Thomas Frank said: "Conor is a top midfielder, who we have worked tirelessly to add to our squad.
"He is still young, so has plenty of room for improvement, but also has huge experience across the Premier League, LaLiga and with the England national team.
"Conor has captained teams so will bring leadership, maturity, character and personality to our dressing room, while his running power, pressing ability and eye for goal will strengthen us in a key area of the pitch.
"I’m excited about working with him every day and I know the fans will love what he will bring to the team."
Gallagher started his professional career at Chelsea, where he was part of the Europa League-winning squad before going out on loan to Charlton Athletic, Swansea City and West Bromwich Albion.
His breakthrough season came during a loan spell at Crystal Palace in 2021/22, where he scored eight goals in 39 appearances and was named the club’s Player of the Season.
He made his full Chelsea ahead of the following season, going on to make 45 appearances across all competitions.
Gallagher featured a further 50 times for the Blues in 2023/24, regularly captaining the side and contributing to 16 goals before his move to Spain.
At Atletico Madrid, he played in both La Liga and the UEFA Champions League, scoring his first European goal in March 2025—just 27 seconds into a round of 16 tie against Real Madrid.
The goal made him the first Englishman to score in a Madrid derby and the fastest English goalscorer in Champions League history.
Internationally, Gallagher was part of the England squad that won the Under-17 World Cup in 2017 and received his first senior call-up in November 2021.
He has since earned 22 senior caps, scored once for the national team, and featured in both the 2022 World Cup and 2024 European Championship, helping England reach the final in Berlin.
Tottenham have also confirmed Gallagher's squad number, with the midfielder taking the number 22 shirt.