Spurs moved quickly to tie up a deal worth £34.7million for the midfielder, who has signed a five-and-a-half-year deal at the Tottenham Hotspur Stadium.
The former Chelsea man returns to the capital - and the Premier League - after a year and a half in Madrid, where he played 77 games for Atleti, scoring seven goals.
“I'm so happy and excited to be here, taking the next step in my career at an amazing club,” Gallagher said.
“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.”
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, La Liga 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 has 21 senior England caps to his name but has not been called up by Thomas Tuchel since the German took charge of the national side.
Spurs were in the market for a midfielder following their latest injury blow, which saw Rodrigo Bentancur ruled out for three months with a hamstring problem.
Frank had gone public with his wishes for reinforcement across the squad, bemoaning his side’s lack of depth in their latest defeat by Aston Villa in the FA Cup.
Lucas Bergvall is also sidelined with injury, while Pape Matar Sarr remains on Africa Cup of Nations duty with Senegal.