Police say a postal worker was approached on Monday in Pierrefonds-Roxboro by an armed suspect, who tried to steal the mail carrier's keys.
Two people were arrested, and one is facing charges.
This is the second such incident to occur in the last few days in the Pierrefonds area.
Police say that on Friday afternoon, another mail carrier was shoved up against the wall of an apartment. They demanded that the worker hand over his keys, before ripping them off his belt.
The two men in their 20s got away on foot.
Police say it is unclear if the two incidents are linked.
There have been about a dozen other similar incidents in Montreal North and surrounding areas earlier this year.
Alain Duguay, president of the Montreal local union, said thieves are hoping to get their hands on cheques and other mail for the purposes of identity theft.
Workers have been told not to resist if approached by robbers.
"Right now, our members are stressed with everything that's happening at Canada Post and having these thieves running after them, that's enough pressure, I think," he said.
Duguay said he hopes the arrests will discourage thieves from targeting mail carriers.
Montreal police said they have stepped up surveillance of Canada Post routes.