
Vertonghen, 33, left Spurs in July after eight years at the Premier League club.
Benfica have also recruited Germany forward Luca Waldschmidt from Freiburg and Brazil winger Everton Soares from Gremio for a combined €35 million.
Both players have signed a contract until 2025 with the Portuguese club.
They represent the first major arrivals since the return of Jorge Jesus as coach last month.
The 66-year-old is back for a second spell in charge, having won three league titles in six years from 2009 to 2015.
Benfica finished runners-up to Porto last season after a poor run of form once the league restarted in June following a coronavirus-induced suspension.