The goalkeeper has left Napoli for 500,000 euros following the transfer of Rafael from Santos and ahead of the expected arrival of Pepe Reina from Liverpool
The Italy international, 36, has joined the Giallorossi on a two-year deal having seen his chances of retaining a first-team position at the Stadio San Paolo plummet this summer.
Rafael Benitez's arrival as head coach in place of Walter Mazzarri, who joined Inter in May, saw Santos shot-stopper Rafael sign for the Partenopei for 5.5 million euros, while Pepe Reina is set to arrive on loan from Liverpool having passed a medical on Wednesday.
A statement on De Sanctis' new club's website read: "AS Roma can announce we have agreed with Napoli for the definitive acquisition of the rights to the player Morgan De Sanctis for 0.5 million euros.
"The footballer signed a contract of employment lasting two years, with effect from the date hereof [July 25] and expiring on 30 June 2015."
De Sanctis joined the Naples-based club in 2009 from Sevilla and made over 100 appearances in a Partenopei shirt.
The ex-Juventus and Udinese man follows the likes of Kevin Strootman, Mehdi Benatia and Maicon as new signings for the Stadio Olimpico side this summer.