Pedro Porro has played once for Spain
The 23-year-old Spain international joined Sporting in August 2020 on a two-year loan deal, which became permanent last summer.
He was previously at Manchester City, but did not play for the first team.
Porro's arrival coincides with Spurs ending the contract of Matt Doherty, who has joined Atletico Madrid.
Fellow defender Djed Spence has also left Spurs, signing for French side Rennes on loan for the rest of the season.
The 22-year-old has struggled for first-team football since signing from Middlesbrough in the summer for £20m. The deal does not include an option to make the move permanent.
Porro, meanwhile, will wear the number 23 shirt for Tottenham.
Signed by Manchester City from Girona for £11m in August 2019, he subsequently had loan spells at Real Valladolid and Sporting before joining the Portuguese club for £7.2m.
He has made one international appearance - in a 2-1 away win over Georgia in a World Cup qualifying tie in March 2021.
Jack Pitt-Brooke, a journalist from The Athletic, told BBC Radio 5 Live Porro was a player Spurs manager Antonio Conte "desperately wanted".
"This is a really big deal for Spurs and maybe one that could improve the mood of the team going into the second half of the season," he added.
"Spurs have been crying out for a top-level right wing-back ever since Conte arrived in 2021 and, frankly, none of Emerson Royal, Djed Spence and Matt Doherty were quite good enough.
"Porro will be a huge lift to the team because he is so much better than what they have got in that position."
-- Courtesy of BBC Sport