TOKYO—Honda and Nissan said they planned to merge in 2026, a move Honda chief executive said wasn’t a rescue of troubled Nissan but a recognition that developing new technologies was too hard to do alone. The companies said they intended to create an automaker that, combined with Nissan partner Mitsubishi Motors, would be the third-largest carmaker in the world, after Toyota and Volkswagen, with more than eight million vehicles sold annually. The announcement came at the end of a year in which Nissan’s profit has tumbled in part because of a dated model lineup in the U.S. Both companies have cratered in China, where electric vehicles and plug-in hybrids are growing quickly.