The Argentine will put pen to paper on a five-year contract with the Italian giant until 2028.
Castellanos spent last season on loan at Girona where he scored 13 goals in La Liga and led the Catalan club to tenth place.
Most notably, he scored a quartet of goals in a 4–2 win against Real Madrid, becoming the first player to do so against Madrid this century.
After impressing in his only season in La Liga, the 24-year-old is now ready for a tougher task at Lazio.
Castellanos, who will mark Lazio’s first summer signing, is seen as the long-term successor to Ciro Immobile, who continues to attract interest from Saudi Arabia.
Speaking of replacing Immobile, it would not be an easy task for Castellanos due to the Italian’s unmatched legacy at the main club.
The Argentine is far from a complete player, but he can master his craft under a tactically strong manager like Maurizio Sarri.
Sarri will be expecting more signings as the top-flight giants play in the UEFA Champions League after finishing runners-up in Serie A last season.
The Biancocelesti are also interested in signing a midfielder in the transfer window, having lost their talisman Sergej Milinkovic-Savic to Al Hilal.
Piotr Zielinski of Napoli, Leandro Paredes of Paris Saint-Germain and Samuele Ricci of Torino were potential targets.