Pioli, who was dismissed as Bologna coach in January, has committed to a one-year deal with Lazio that would be extended automatically if the club qualify for the Champions League or Europa League.
"The club had offered me a two-year deal, but they have accepted my proposal of it being an annual one that renews automatically if I manage to guide us into Europe," Pioli said at a news conference on Friday. "I'm happy, proud and honoured to have been given this new job. I will give it all I've got to ensure next season will be highly positive."
Pioli officially starts work on July 1, but he is not going to waste any time putting his stamp on the club and has already effectively started work.
"There will be changes," he said. "The players who will be part of my squad have got to have technical and moral quality to be here. I want a team that has the eye of the tiger and is foaming from the mouth, one which takes to the field convinced they can get the three points."
Lazio finished last season in ninth place, just two points shy of a place in the Europa League next season.
Reja, 68, informed club president Claudio Lotito on Thursday that he would not be renewing his contract.