Benatia, 26, has signed a five-year contract at the Stadio Olimpico following three seasons with Udinese in which he made 72 league appearances, scoring five goals.
With permission from his former club, Benatia was allowed to train with his new team-mates in Trigoria on Saturday before the deal was ratified.
A statement published on asroma.it also confirmed the transfer of midfielder Valerio Verre and striker Nicolas Lopez, both 19, to Udinese.
Roma have received 2.5million euros for Verre with the sale of Uruguayan Lopez netting the capital club one million euros.
Verre failed to make a senior appearance for the Giallorossi but Lopez scored a late equaliser in a draw with Catania on his league debut last August.