The goalkeeper has left the Partenopei for £431,000 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 [£4.7m], 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.
"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 has made over 100 appearances in a Partenopei shirt.
The ex-Juventus and Udinese man follows the likes of Kevin Strootman, Mehdi Benatia and Maicon to transfer to the Stadio Olimpico side this summer.