Retegui set an Atalanta scoring record in a Serie A season when his 25th goal snatched a 3-2 win at Genoa.
The previous record was 24 by Filippo Inzaghi in 1996-97.
Retegui is also waiting to be crowned the league's top goal-scorer this season, as he has seven more than his nearest challenger.
He is in his first season at Atalanta after joining from Genoa in August.
It was the only match in the penultimate round with only pride to play for. The other nine matches kick off at the same time on Sunday, when Napoli could win Serie A.
Atalanta had already secured third spot in Serie A, and Genoa were comfortably mid-table.
Genoa took the lead when Andrea Pinamonti headed an Aaron Martin cross to end a three-month goal drought.
Ibrahim Sulemana leveled for Atalanta immediately after the break with a fine strike from distance.
Pinamonti doubled his tally in the 58th, squeezing a finish into the bottom near corner after some woeful Atalanta defending.
Daniel Maldini hauled Atalanta level again five minutes later after a Marten De Roon through ball was allowed by a smart Retegui dummy.
Atalanta turned the match around completely with a minute remaining when Charles De Ketelaere raced down the right before rolling across for Retegui to sweep home.
The Italy forward didn't celebrate against his former team.
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