The Serb becomes the fourth full-back to leave City this summer following the release of Pablo Zabaleta, Gael Clichy and Bacary Sagna.
Pep Guardiola's side today completed the signing of Brazilian right-back Danilo on a five-year deal for an undisclosed free.
Kolarov's move to Roma should pave the way for Monaco's French international full-back Benjamin Mendy to join City.
Mendy is expected to link up with City in LA for a medical and to complete a £52m move.
31-year-old Kolarov told the Manchester City website that he had thoroughly enjoyed his time at the club and that he was sad that it had to come to an end.
"When I came in 2010, honestly, I didn’t expect to play so long," he explained.
"Today is the day I have to say goodbye. Everything was quick the last couple of days. For me, it’s a very sad day."
"In every part of the Club we have improved not 100 per cent but 200 per cent. Now we are a far better Club than what we were when I came. I am proud that I was part of making City the Club that it is."
"For me it was a great pleasure to play for this shirt."
UFFICIALE: Aleksandar #Kolarov è un calciatore dell'#ASRoma
— AS Roma (@OfficialASRoma) 22 July 2017
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