Gallagher has 22 caps for England but has played only once under current boss Thomas Tuchel, featuring for 59 minutes in a 3-1 friendly defeat by Senegal.
He will hope a move back to the Premier League and impressing in Thomas Frank’s side will boost his chances of making England’s squad for this summer’s World Cup.
Gallagher was an unused member of the squad at the 2022 tournament, but made five appearances during the 2024 Euros campaign, where the Three Lions were beaten in the final by Spain.
“Conor is a top midfielder, who we have worked tirelessly to add to our squad,” said Spurs boss Frank.
“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.”
Spurs had been looking to add to their midfield options after Rodrigo Bentancur was ruled out for at least three months.
The Uruguay international needs surgery after sustaining a hamstring injury in the 3-2 Premier League defeat by Bournemouth last week.
Spurs are 14th in the table on 27 points after 21 games and have had a run of poor form in which they have won just one of their last six league matches.
They remain in contention for automatic qualification for the knockout phase of the Champions League, with Spurs currently 11th in the standings, but they were beaten by Villa in the third round of the FA Cup last Saturday – a seventh defeat in 13 games across all competitions.
They next host strugglers West Ham in the Premier League on Saturday (15:00 GMT).
Gallagher started his career at Chelsea as a youngster in 2006 and during his professional career there he had loan spells at Charlton, Swansea, West Brom and Crystal Palace.
He made 136 Premier League appearances in total – for Chelsea, West Brom and Palace – before joining La Liga side Atletico, who he represented 77 times across all competitions, scoring seven times.
