Rulli - who was part of the Argentina squad which reached he 2026 FIFA World Cup final - accepted an offer to join City from French club Olympique de Marseille on a two-year contract.
The 34-year-old is expected to slot in as a replacement for James Trafford, who left City to join Leeds United, with the veteran acting as back up to No.1 Gianluigi Donnarumma.
Rulli returns to City after spending the 2016/17 season at the club without making a senior appearance before moving on to LaLiga side Real Sociedad.
He's expected to be included in City squads from September onwards, as he builds up his fitness after an extended summer break, and he's keen to show his best version and challenge Donnarumma if the chance comes along.
"Everyone I spoke to about City said I had to join them because they are a club with incredibly high standards.
"Every professional wants to be in a high-performance environment and I can't wait to get started because I know I will improve and learn, which is always my main priority.
"My job is to train hard, get to know the players and staff and do everything I can to reach my best level."
City open their Premier League season at home to Bournemouth on August 23rd.