The board's decision Thursday allows Enbridge to move 300,000 barrels of crude oil per day from Sarnia, Ontario, to Montreal.
Line 9 originally shuttled oil from Sarnia to Montreal, but was reversed in the late '90s in response to market conditions to pump imported crude westward. Enbridge now wants to flow oil back eastwards to service refineries in Ontario and Quebec.
The capacity of the line will also increase to 300,000 barrels a day from 240,000 barrels.
Opponents argued the plan puts communities at risk, threatens water supplies and could endanger vulnerable species in ecologically sensitive areas.