SEOUL, South Korea — North Korea said Wednesday that it would open the seventh congress of its ruling Workers’ Party next week. It will be the country’s biggest political convention in more than three decades and a likely platform for its leader, Kim Jong-un, to cement his authority.

The party faced a “task of advancing the final victory of its revolution under the leadership of Kim Jong-un,” its Politburo said, announcing that the congress, the first since 1980, would begin next Friday.

Mr. Kim already oversees the party as its first secretary. In recent weeks, towns and provinces in North Korea have held party meetings to elect delegates to be sent to the congress in Pyongyang, the capital. They all adopted a motion to uphold Mr. Kim as the party leader.