Police spokesman Jean-Pierre Brabant said the victim is a man is his fifties with mental health issues.
The victim was making purchases at a convenience store at the corner of De Lorimier Avenue and Des Carrières Street. Brabant says he was carrying an envelope containing hundreds of dollars which he laid down on the counter.
"While he was doing that purchase, another man kind of got into his face, grabbed the envelope that contained the money, and left with it," said Brabant.
He said the suspect then left in a car driven by another person.
Brabant said investigators have no leads after interviewing the victim and other witnesses, so they are releasing images from a surveillance camera of the suspect and the car.
The suspect is a black man between 25 and 30, five foot eight and about 165 pounds. At the time of the robbery he was wearing a black leather jacket and a dark hooded sweatshirt.
The vehicle is a grey Nissan Maxima four-door with with tinted windows and a sunfroo, model years between 2004 and 2006, possibly with a broken side mirror on the driver's side.
Police are asking anyone with information to call the Info-crime line at 514-393-1133.