City manager Roberto Mancini only claimed on Monday that no players would be brought in during the final days of the transfer window, but has been given the opportunity to add depth to his midfield.
The Chilean international has signed until the end of the season as he links up with former manager Mancini who was his boss for one year after joining Inter in 2005, winning a Serie A title, the Italian Super Cup and Coppa Italia with Inter.
Pizarro has struggled for first-team football at Roma this season since former Barcelona man Luis Enrique became head coach, appearing just seven times, but the deep-lying playmaker adds experience and European pedigree to the City midfield.
The midfielder had been linked with Genoa, Juventus and Zenit St Petersburg in recent weeks, but City raced to the front of the queue late on, but due to his lack of action this season, it is likely that he will have to work up his fitness before getting into the City side.
The former Udinese man made 148 league appearances at Stadio Olimpico since signing from Inter in 2006 and played 22 Serie A matches last season as his influence on the side decreased.
Elsewhere at City on deadline day, young Swiss defender Frederic Veseli joined Manchester United and left-back Wayne Bridge moved on loan to Sunderland.