Newcastle announced on Monday that Yanga-Mbiwa arrived at Rome's Fiumicino Airport to undergo a medical and finalise the deal.
Roma statement: "An agreement has been signed with Newcastle United to acquire the player Mapou Yanga-Mbiwa on a temporary basis, until June 30, 2015, for a payment of £1million.
"The agreement also includes the option to buy the player outright, from the 2015/2016 season, for a total value of £5.5million."
He arrived from Montpellier for £6.7 million
The 25-year-old had been looking to leave the Magpies all summer but Alan Pardew said he wouldn't do a deal until he signed another central defender.
As it happens, he failed to bring in a replacement.