The pipeline, which traverses Canada's two westernmost provinces, was "safely restarted," Trans Mountain said.
Trans Mountain said there had been "no indication of any product release or serious damage to the pipe" during the shutdown, but that it had been closed protectively.
The underground conduit carries 300,000 barrels of petroleum a day from the oil sands of Alberta to a maritime terminal in a Vancouver suburb.
The shutdown, prompted by major storms in mid-November, led to gas shortages in southwestern British Columbia.
The provincial government there was forced to impose fuel rationing: individuals could buy no more than 30 liters (eight gallons) of fuel at the pump.
Rationing, however, will remain in place until mid-December as supplies are restocked, the government said.
Trans Mountain, in operation since 1953, is the only petroleum pipeline linking Alberta, the biggest oil-producing province, to Canada's Pacific coast.
The federal government of Justin Trudeau purchased the pipeline in 2018, amid strong opposition from environmentalists and indigenous communities to a proposal to triple its capacity.
