Lazio confirmed the departure through an official statement, saying they had transferred the Senegal international on a temporary basis with the purchase becoming mandatory once certain sporting conditions are met during the season.
The structure allows Lazio to bank a fee gradually while limiting the immediate impact on Rennes's books, with reported figures putting the deal at around €2 million euros upfront and a further payment due once the obligation is triggered.
Dia joined Lazio in 2024 following his release clause being paid to Salernitana, and made 81 appearances for the Rome club across all competitions, including 12 in the Europa League as Lazio reached the quarter-finals of that competition in April 2025.
He also featured in a Coppa Italia final during his time in the capital, though he was unable to establish himself as a guaranteed starter under head coach Rino Gattuso this season.
The move returns Dia to Ligue 1, where he previously enjoyed a prolific spell at Stade de Reims before his switch to Italy via Salernitana. Personal circumstances are understood to have played a part in his decision to prioritise a return to France, alongside the search for more regular playing time than he had been able to find in Rome.
Lazio have already brought in Albert Gudmundsson and Andrea Pinamonti as attacking reinforcements this summer, meaning Dia's departure completes a broader reshaping of the club's forward options heading into the new season, with Petar Ratkov also having left the club on loan to Levante on the same day.
The overall value of the deal has been reported differently across Italian outlets, with some putting the total package as high as €9-10 million once the obligation clause activates, reflecting the staggered nature of the payment structure agreed between the two clubs.
For Dia, the move offers a fresh start in a league where he previously thrived, and a chance to reestablish himself as a first-choice forward after a season in Rome that never quite delivered the consistent playing time he was hoping for.