Through their official website, Roma declared Marcano signed on a free transfer for three years or until 30 June 2021 with the Giallorossi.
Previously, the 30-year-old Marcano defended Portugal's Porto giants for the past four seasons.
"I am very happy to be here, because Roma is a great club and moving in the right direction with a clear sports project," Marcano said.
"I also believe that this team's soccer philosophy will match my strengths and characteristics," added the Spaniard.