Workers in three groups voted in favour of the tentative deal with the aluminium company by margins of 82 per cent to 92 per cent.
The 780 Rio Tinto Alcan workers were locked out on Dec. 30.
One of the key issues in the dispute was outsourcing. The international company wanted to replace retiring workers with sub-contractors that would be paid lower wages.
The union said Thursday night that the new contract limited outsourcing, bolstering the interests of the Quebec employees.
During the dispute, the union had also denounced the company's continued operation of its dams and the selling of its electricity production to Hydro-Quebec.
"We faced the third-largest mining company in the world and we won," said Leo Gerad, the international president of the United Steelworkers.
"The message to multinational corporations is clear: workers are more determined than ever to defend their rights."
The new contract runs until Dec. 21, 2015. Details of the deal will be announced on Friday, the union said.
"I thank our members and their families have never abandoned despite six months of lockout, said Mark Maltais, president of Local 9490.
"We can go back to work with heads up."
The company welcomed the positive vote Thursday night.
"This is excellent news for our employees, their families, the community, our customers and our shareholders," said Etienne Jacques, CEO for the North American division of Rio Tinto Alcan Primary Metal.
"The agreement will help protect the competitiveness of the Alma plant in the future."
The company said management will being the back to work process for the employees over the next few days, followed by the restart of the plant smelter's production cells.
The new deal was reached Sunday with the intervention of a Ministry of Labour conciliator.
The Alma plant is in Quebec’s Saguenay region, about 225 kilometres north of Quebec City. It is one of Rio Tinto Alcan’s key aluminum smelters, producing about 438,000 tonnes of aluminum a year.
The lockout caused a one-third reduction in output at the smelter.
Alma's mayor had said the lockout hit the local economy hard.
Marc Asselin had said a positive outcome would bring welcome relief to the community, which has been watching its pennies over the last few months.
"In the last six months, people have been careful with their expenses, careful with their investments because they didn't know when the conflict would end."
Kathleen Voyer, director general of the Lac-St-Jean-Est chamber of commerce and industry, said that prudence was clearly evident.
"We saw a slowdown on the construction side, also in renovations and at the level of the car dealers," she said. "It was the same thing in the beauty treatment sector, for example. These were the sectors that were affected more than the others."
Asselin said economic restraint was also exercised by the town, which was careful about how it spent its funds.