The Montreal transit corporation said the problem was due to a broken down metro train at the Bonaventure station.
Service is now gradually returning to schedule.
There was initially some confusion about the extent of the outage, with conflicting reports coming from the STM's spokesperson and the service's Twitter feed.
This is second time this week a technical problem led to a breakdown on the metro.
On Monday, service on a large chunk of the green line was suspended for more than two hours because of an electrical problem.