The International Longshore and Warehouse Union said 88 per cent of its members voted to ratify a contract running through May 2018. It includes work rule changes and raises.
The grain handlers association says the agreement is fair and would allow the companies to remain competitive.
Louis Dreyfus Commodities, United Grain and Columbia Grain operate grain terminals in Portland, Seattle and Vancouver, Washington. More than a quarter of all U.S. grain exports move through nine terminals on the Columbia River and Puget Sound.
Two years of negotiations, lockouts and occasional violence centred on the companies' efforts to get management-friendly work rules to match those at other area terminals.