The move was completed as a loan with a conditional option to buy, rather than the mandatory obligation initially agreed, after Lazio's spending rules forced a late change to the structure of the deal.
The club had wanted to finalise the transfer as a loan with an obligation to buy set at €15m, but discovered their available budget under Serie A's financial regulations stood at only €13m — enough to complete the move, but not enough to register Frattesi to play until the gap was closed, either through fresh investment from ownership or a separate departure.
Rather than wait, Lazio and Inter Milan agreed a new formula: a €5m loan fee with a €10m option to buy, rather than an obligation, along with a 50% sell-on clause retained by Inter on any future transfer.
Both clubs confirmed the move in official statements.
"S.S. Lazio announces it has acquired on a temporary basis, with an option for a permanent transfer, the services of footballer Davide Frattesi from F.C. Internazionale," Lazio said, while Inter's statement confirmed "the transfer of Davide Frattesi to S.S. Lazio" on a temporary deal "with an option in favour of the Rome club."
Frattesi, 26, grew up in Lazio's youth academy before later joining Roma's, and spoke emotionally about the move on Lazio Style Channel after landing in Rome and completing his medical and paperwork.
"It's been an odyssey," he said of the day itself. "On days like this, the waiting increases the desire, even if it's tiring."
Asked whether the move felt like a homecoming, he said: "Yes, definitely. When I arrived, I recognised all the streets I used to take when I trained at Giustinianese. I'm coming back with a different maturity — at 26 I have to take on real responsibility."
He admitted he had needed to feel wanted again after a difficult recent spell at Inter.
"I needed to feel loved. I had this relationship with the fans at Inter too, always got on with them through ups and downs, but I needed to feel back at the centre of a project."
He revealed the move had come together in a rush after an earlier attempt collapsed in January.
"We were close in January, then for various reasons it didn't happen. This year I was told two days ago, so I only had time to say yes and sort out the paperwork."
Frattesi described his father as a lifelong Lazio supporter who took him to matches as a boy, and said he was looking forward to working again with head coach Gennaro Gattuso, whom he knows from his time in the Italy squad.
"I've heard more from him these past few days than from my own girlfriend," he joked. "I'm glad to be working with him again — I know how he works, I know what kind of person he is."
On the new midfield trio of himself, Nicolò Rovella and Kenneth Taylor, Frattesi was cautiously optimistic.
"This is a good midfield on paper, but it has to be proven on the pitch. We'll talk about it again in a few weeks."
He also confirmed hopes of a return to the Italy squad, saying national team coach Roberto Mancini had told him that what mattered most was playing regularly, and that Mancini's message had played a part in his decision to move.