The 23-year-old has signed a five-year contract, worth a reported £200,000 a week.
No fee was confirmed by either club, but it is thought City have paid Atletico Madrid £35million for Aguero, eclipsing their previous high of £32.5million for Brazil forward Robinho in August 2008.
He becomes manager Roberto Mancini's third signing of the summer, following the arrivals of Arsenal defender Gael Clichy and Partizan Belgrade's Stefan Savic, and takes the Italian's spending in this window to £48million.
Source: PA
Source: PA