The former Lyon midfielder has been reunited with Rudi Garcia after signing for the Giallorossi on a temporary basis
The Giallorossi have the option of making the transfer permanent in the summer with a fee of €3.5 million (£2.9m) agreed with the UAE club.
The Brazilian midfielder signed for Al Ain from Lyon in August after a stint on loan at Schalke, but has now opted for a move back to European football.
Bastos has previously worked with current Roma coach Rudi Garcia at Lille, and will provide competition in wide areas as the club look to challenge Juventus at the top of Serie A.
The 30-year-old has been given the number 20 shirt at the Stadio Olimpico and could feature in Roma's next Serie A fixture at Verona on Sunday.