The US aluminium group had planned to close the plant by the middle of the year.
Under the agreement, Portovesme workers would go into a temporary layoffs procedure from November if no offer to take over the plant arrived, the FIM-CISL metal workers' union said in a statement.
There was no immediate comment from Alcoa.
Alcoa said in January it would close the Portovesme smelter as part of its efforts to cut global output and costs. The plan has run into fierce opposition from Italy's government, Sardinia's authorities and trade unions.