The about 800 workers, from three separate bargaining units, have been locked out since January. Subcontracting of labour to replace retiring workers has been the main stumbling block in negotiations.
The lockout went into affect after talks collapsed just hours before the workers' contract expired.
The deal was reached Sunday with the intervention of a Ministry of Labour conciliator.
"We are satisfied with the agreement," said Etienne Jacques, CEO of North American operations for Rio Tinto Alcan Primary Metal. "Both parties remained open, which enabled us to reach an agreement."
The workers, members of members of Steelworkers Local 9490, will vote on the deal at a general assembly over the next few days.
In a release issued Monday, the union said representatives will not be commenting on the tentative agreement pending the membership meeting and ratification vote.
The Alma plant is in Quebec's Saguenay region, about 225 kilometres north of Quebec City. It is one of Rio Tinto Alcan's most important aluminum smelters in North America, producing 438,000 tonnes of aluminum a year.
The lockout resulted in a one-third cut cut in production at the smelter, with about 200 managers doing the work.
In the early days of the dispute, the union accused the company of hiring illegal replacement workers. The company, in turn, sought an injunction to limit the number of workers petitioning outside the plant to 20.