Candreva was co-owned by former club Udinese, and had been linked with the likes of Paris Saint-Germain and Juventus, though Lazio president Claudio Lotito this week ruled out an exit for the Italy international.
The 27-year-old - who has been at Stadio Olimpico since January 2012 - has now been bought outright by the Rome outfit.
A statement on the club's official website read: "Lazio announces that it has resolved in its favour the signing of the player Antonio Candreva."
Candreva is currently in Brazil where he is a member of Italy's FIFA World Cup squad.
"The club wishes to confirm that in the next few days we will announce the acquisition of player Dusan Basta," the statement continued.
Lazio - who this week announced Stefano Pioli had replaced Edoardo Reja as head coach - finished ninth in Serie A last season.
