City confirmed the deal on their official website as the defender makes his way to Serie A.
The 31-year-old departs City as manager Pep Guardiola shuffles the pack ahead of the new campaign.
Sky in Italy (h/t Rory O'Callaghan of Sky Sports) reported the capital city giants had a £5.8 million offer for the Serbia international accepted on Friday.
Speaking to City's website, Kolarov explained his feelings as he leaves the English team:
"When I came in 2010, honestly, I didn’t expect to play so long. 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."
The full-back featured at the heart of Guardiola's defence last term, as the former Barcelona boss implemented his favoured formation at the Etihad Stadium.
Kolarov played in 35 games in the Premier League and UEFA Champions League last season, according to WhoScored.com.
The player arrived at City after a spell in Italy with Lazio, signing in 2010 after three years in Rome.
Kolarov is a two-time winner of the Premier League with City, also adding the FA Cup to his achievements in 2010.
The Serbian is known for his powerful shot and excellent economy from set pieces. The pace of the Italian game will suit Kolarov in the latter years of his career.
Roma will appreciate the full-back's experience, and he can play in a number of positions.