The incident happened around 5:30 a.m. along Highway 132 near St-Lambert, south of Montreal.
Provincial police spokeswoman Sgt. Mélanie Dumaresq said the tanker driver ended up in the river after trying to avoid two vehicles that had just collided and were stopped at the side of the highway.
The tanker side-swiped the two vehicles and crashed into the river.
Its driver was able to get out of the truck with the help of two people in a boat, and Dumaresq said no one was injured.
The truck was carrying dry cement, and a team of environmental experts is on site to manage the cleanup.
Two westbound lanes of Highway 132 are closed as a result of the incident, backing up morning commuter traffic.
"We're trying to get traffic moving again as fast as we can, but we have to make sure there's no gas leak as the tanker is removed from the water," Dumaresq said.