SEOUL—With denuclearization talks between the U.S. and North Korea stalled, South Korean President Moon Jae-in is set to visit Pyongyang on Tuesday in a bid to revive diplomacy after a summer of challenges.

At a planned three-day summit, Mr. Moon and North Korean leader Kim Jong Un are expected to hold talks on a declaration to formally end the Korean War, more than six decades after fighting was halted by an armistice.

North...