Reja confirmed his departure on Thursday after failing to secure UEFA Europa League football for next season, with Lazio ending the Serie A campaign ninth.
The Italian had been in charge of the Rome-based club since January, returning for a second spell after Vladimir Petkovic was sacked.
Pioli joins Lazio after suffering relegation with Bologna and ultimately losing his job.
The 48-year-old, a former Juventus and Fiorentina centre-back, has had numerous coaching jobs since hanging up his boots in 1998, including spells with Parma, Chievo and Palermo.
Pioli took over at Bologna in October 2011 and guided the club to ninth and 13th-placed finishes, but the team could only accumulate 29 points last season and dropped down to Serie B.
The club will officially present Pioli as their new coach at a press conference on Friday.
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