Hart was quickly deemed surplus to requirements at City following Pep Guardiola's arrival last summer and eventually joined Serie A outfit Torino. The 30-year-old made 37 total appearances for I Granata, keeping only five clean sheets as Sinisa Mihajlovic's side secured a ninth-place finish.
The chances of Hart returning to City on a full-time basis appeared extremely remote and Guardiola's decision to spend £35m ($45.6m) on Benfica custodian Ederson only served to reinforce the notion that he had no future under the Catalan.
The BBC report that West Ham are paying a £4.5m loan fee for Hart, with City agreeing to subsidise part of his wages. The deal is not thought to contain an option to buy.
"It's important for me to play football and to get the opportunity to come here and play is absolutely awesome," Hart told West Ham TV after his move was finally announced on Tuesday (18 July). "To be playing in the Premier League for a big club like West Ham, I'm really looking forward to the season.
"I've always loved the club. I think you'll struggle to find anyone, apart from the direct opposing fans around London, who doesn't love the club. It's got something about it, it's got a lot history and I've always loved playing against West Ham."
Hart will reunite with Pablo Zabaleta at West Ham after the Argentine right-back penned a two-year deal with the club in June following his release by City.
With their second summer signing now secured, the Hammers are expected to push on with their desperate pursuit of two new strikers. The Evening Standard understand that a club-record £23m deal for wantaway Stoke City forward Marko Arnautovic should be completed during the next 48 hours.
West Ham have also been closely linked with former Manchester United striker Javier Hernandez, who currently plies his trade in Germany with Bayer Leverkusen. Sky Sports report that they also hope to complete that particular signing this week, with Chicharito's current contract said to contain a release clause valued at just £13m.