The Aquile endured a difficult 2013-14 campaign, finishing ninth and missing out on European qualification, and it was announced on Thursday that Reja would be leaving the club despite only arriving in January following the sacking of Vladimir Petkovic.
Rumours had circulated since the end of last season that the 68-year-old would step down from his role at the club after disappointing results and fostering a poor relationship with supporters and Lazio have ultimately chosen Pioli as his successor.
"SS Lazio announces that Stefano Pioli will be the new coach of the first team from 1 July 2014," a statement on the club's official website read.
"A press conference presentation is scheduled for tomorrow at 12:00 at Sports Centre of Formello."
Pioli was sacked by Bologna in January after leaving Rossblu one point above bottom spot in Serie A.
His appointment came only hours after it was revealed that Reja had left his position as Biancocelesti president Claudio Lotito looks for the club to challenge for European qualification next term.