Service was down between Atwater and Pie IX stations from roughly 5:45 a.m. to 8:20 a.m.
The city's transit corporation had discharged about 40 shuttle buses to run parallel to the green line and people were also encouraged to use the metro's orange line as an alternate route downtown.
While service has been restored, commuters could still be facing some delays.
At the Lionel-Groulx station, the lineup for shuttle buses was at least two blocks long when an official announced to commuters that the metro was running again.
The extra shuttle buses were immediately halted, forcing all those waiting in line to go back inside the station.
The outage was due to an electrical problem, according to STM spokeswoman Isabelle Tremblay.
There was initially some confusion with reports from the STM's spokesperson conflicting with the information from the service's Twitter feed and announcements being broadcast in the metro.
In 2012, Montreal's metro system had more than 1,000 unplanned stoppages, making it the worst record for service delays in six years.
On Dec. 9, the service was down on the entire network for about 45 minutes. The STM said that was due to a computer problem.