Frattesi will join on a loan worth €1m with an obligation to buy set at €14m, triggered either by the midfielder reaching 25 appearances this season or Lazio finishing in the top half of Serie A.
Inter Milan have also secured a substantial 50% sell-on clause on any future transfer, along with a further €3m in bonuses, protecting their long-term interest in a player they signed for a considerable fee three years ago. Frattesi will sign a five-year contract in the capital — one on loan, four permanent — on wages of around €3m a season, matching what he earned at Inter.
The move ends Frattesi's three-year spell in Milan and reunites him with a club that first showed serious interest back in January, before Inter opted to keep hold of him at the time. Head coach Gennaro Gattuso pushed hard for the deal as he looks to strengthen his midfield options ahead of the new season.
With Frattesi's arrival now confirmed, Lazio's attention has turned fully to reinforcing the attack.
Sporting director Angelo Fabiani has held informal talks over free agent Mauro Icardi, who left Galatasaray at the end of June after winning the Süper Lig title, though the move remains no more than a sounding at this stage.
The 33-year-old, who scored 121 goals in 219 Serie A appearances for Inter and Sampdoria and won two capocannoniere titles, is weighing offers from Saudi Arabia, Qatar, Brazil, Argentina and Mexico, and his wage demands — believed to be significantly higher than Lazio's usual parameters — represent the biggest obstacle to any return to Italy.
A move for Sassuolo's Andrea Pinamonti is considered the more realistic route to a new striker. The forward has already given his approval for a switch to Rome, having scored 48 goals across his last five Serie A seasons between Genoa, Empoli and Sassuolo, with Lazio proposing a loan worth €3m and an obligation to buy set at €12m, conditional on performance-related targets.
Lorenzo Lucca of Napoli remains a background option should the Pinamonti or Icardi routes fail to progress.