It was announced by Sydney FC that Colosimo had signed a two-year deal, and he joined the club from the start of the 2008–09 season. Colosimo became a standout performer as a centreback, with consistent performances impressing through the new 2009–10 season. On 31 March 2010, the defender switched clubs to Melbourne Heart, along with his teammates Clint Bolton and John Aloisi. He became the captain of the new club and scored one goal in their debut season, an important equaliser against Adelaide in the final minutes of the game, which Heart triumphed 2–1 in stoppage time.