The former Arsenal captain, 30, left north London to join Barcelona for £15m in 2014 but injuries have restricted him to 11 appearances in La Liga.
Roma have an option to buy the Belgium international at the end of the season.
Vermaelen's move means that £25m-rated Greece defender Kostas Manolas, who has been linked with several Premier League clubs, could leave the Stadio Olimpico.
Barcelona defender Thomas Vermaelen has joined Serie A side Roma on a season's loan.