The 32-year old Argentine centre-back has signed a two-year deal with City for a reported fee of £3.5million after only joining Atletico from Malaga earlier this summer and is looking forward to reuniting with his ex-Los Boquerones and River Plate boss Manuel Pellegrini.
"I am delighted to have joined City because I am still hungry for success and want to improve," he told the club's official website.
"I have been in Europe for 11 years now and this was a great chance for me to play in the Premier League with such a big club.
"I have lived in Europe for many years and I don't think there'll be any problems in adjusting to life here.
"Now I want to repay the manager's faith in me out on the pitch. I can't wait to get started."