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.
When he returns he is likely to be joined by Basta, with Lazio announcing that a deal to bring the 29-year-old from Udinese is imminent.
"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.
