The 29-year-old has been pushed down the pecking order under new manager Pep Guardiola and has not started a Premier League game this season.
Willy Caballero has been first choice and with Claudio Bravo's arrival from Barcelona for £13.75m. Hart has elected to move to get regular first-team action.
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"Suddenly my career has undergone a change, I interpreted it as a sign of destiny," he told the Torino website.
"The move to Torino has come at the right time, in the right way.
'I'm very happy to be able to test myself in a beautiful and difficult league like Serie A.
"Your goalkeeping tradition is recognised throughout the world, so I'm convinced I can learn a lot here at Toro.
"For that reason, I immediately accepted president Cairo's offer with great enthusiasm. In terms of the staff, I already know Lombardo.
"I worked together with Attillio in Manchester and we won, we hope to relive a lot of wonderful emotions together.
"I read some interviews from Toro... the coach [Sinisa Mihajlovic] wants humble but ambitious players - I also really like that, that's how I want to be."
The move comes after England manager Sam Allardyce insisted that the goalkeeper would be in the squad to face Slovakia in a World Cup qualifier on Sunday, but would wait to see what his mindset would be before deciding whether to start him.
Hart flew to Turin on Tuesday and was pictured with fans. The Italian club are reportedly set to pick up £55,000 of his £135,000-a-week wages.
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