The Portugal international posted a picture on Instagram on Thursday evening, which showed a cut above his right eye.
The 27-year-old full-back did not say where the attack took place, but City said in a statement it was during a burglary at the player's home.
"Unfortunately today I was assaulted by four cowards who hurt me and tried to hurt my family," Cancelo posted.
"When you show resistance, this is what happens. They managed to take all my jewellery and leave me with my face with this state."
Cancelo added: "I don't know how there are people with such meanness.
"The most important thing for me is my family and luckily they are all OK.
"After so many obstacles in my life this is just one more that I will overcome. Firm and strong, like always."
Cancelo, who joined City from Juventus in August 2019, is widely considered to have been one of the best players for Pep Guardiola's Premier League leaders this season.
As an 18-year-old, the Portuguese player was a passenger in a car accident that killed his mother.
A Manchester City statement read: "We are shocked and appalled that Joao Cancelo and his family were subjected to a burglary at their home this evening during which Joao was also assaulted.
"Joao and his family are being supported by the club and he is helping the police with their enquiries as they investigate this very serious matter."