The youngster will join the Catalan side's B team on a three-year contract with a 12 million euro release clause and becomes the Blaugrana's first summer acquisition.
The 19-year-old will join the Catalans' B team on a three-year deal with a 12 million euros release clause, and becomes new boss Tito Vilanova's first signing at the club.
An attacking midfielder by trade, Roman joined City from Espanyol three years ago in a 1.25m euros deal but never featured in Roberto Mancini's first team.
“Being in England for three years has helped me to mature a lot and to become physically stronger," he told Barcelona's official website.
“I’m raring to start and start learning. When I heard Barca were interested in me, that was my priority. I can assure you that I’ll work hard and give everything I can out on the pitch."
He also took to his Twitter account, saying: "[I] would like to give a special mention to all [the] City fans for the support you have given me, which will always stay with me. Thank you all.
"I would like to announce that I will be playing at Barca B for the next three years. It hasn't been an easy decision to leave Manchester City where I have spent 3 wonderful years.
"But all in all Barca is the best option to continue my career right now."
Born in Reus, Catalonia, Roman has represented Spain at all international youth levels.