Florenzi becomes Stefano Pioli's latest acquisition in a busy transfer window, with Milan confirming they have an option to sign the full-back on a permanent deal.
The 30-year-old had not featured for Roma since January 2020, spending time on loan at Valencia and then Paris Saint-Germain, where he played 21 times in Ligue 1 last term.
Despite spending the previous season at the Parc des Princes, only Daniel De Rossi (20,370) and Edin Dzeko (19,686) have played more minutes for Roma than Florenzi's 18,925 since his debut in May 2011.
In the same time period, only Radja Nainggolan (285), Lorenzo Pellegrini (296), Dzeko (312), Francesco Totti (366) and Miralem Pjanic (403) have created more chances for Roma than Florenzi, who tallied up 282.
Among defenders in Serie A, Alex Sandro (321) and Aleksandar Kolarov (295) are the only two to have created more than the full-back, who will be back plying his trade in Italy.
A part of Italy's successful Euro 2020 squad, Florenzi also has experience on the international stage, too.
He appeared twice for Roberto Mancini's side as they secured the trophy, including a brief cameo in the final against England at Wembley, to take his total to 45 caps since making his debut in 2012.
Milan get their Serie A campaign underway on Monday when they travel to Sampdoria.