The Italy international, who joined City from PSG, made his preference clear in an interview with L’Équipe.
“For me it’s Ousmane Dembele,” Donnarumma said simply when asked who should claim this year’s prize.
Donnarumma and Dembele developed a strong relationship during their time together at the Parc des Princes.
The pair also faced each other as international opponents in the UEFA Nations League on Friday night, with Italy taking on France.
Dembele won the Ballon d’Or last year after an outstanding campaign with PSG and is among the contenders again.
However, the race is widely regarded as wide open, with Barcelona teenager Lamine Yamal emerging as a strong favourite following his impressive form for club and country.
Real Madrid’s Mbappe and Bayern Munich’s Kane are also firmly in the conversation, alongside Lionel Messi.
Yamal is viewed as the strong favourite in some quarters, with many pointing to his World Cup win as the deciding factor over Kane and Mbappe.
Despite that, Dembele, who won a second straight Champions League with PSG, will be hoping his former teammate’s endorsement adds momentum to his bid to retain the prestigious individual honour when the winner is announced later this month.