Financial details were not disclosed but British media reported that Spurs paid around 40 million pounds for the defender.
Porro becomes the north London club's second signing of the January transfer window after they also brought in winger Arnaut Danjuma (26) on loan from Villarreal.
Manchester City signed Porro in 2019 but immediately loaned him to Real Valladolid for one season before he joined Sporting on a two-year loan deal. In 2022, Sporting triggered Porro's buyout clause and signed him on a permanent deal.
Porro joins Tottenham as they look to strengthen the right side of their defence and fight for a top-four spot, with Antonio Conte's side currently fifth on 36 points after 21 games. They next host champions Manchester City on Sunday.
Doherty joins Atletico Madrid from Tottenham
Porro's arrival followed the departure of right back Matt Doherty (31) to Atletico Madrid after the Irish player agreed to terminate his contract with Tottenham.
Doherty joined Spurs from Wolverhampton Wanderers in August 2020 and made 71 appearances, scoring three goals in all competitions.
Atletico Madrid also signed Dutch striker Memphis Depay (28) in the January transfer window.