Qong Tayiji (also spelled Khong Tayiji; in Manchu: Hong Taiji) is a title of the Mongols.
Qong Tayiji derives from Chinese Huangtaizi (皇太子; crown prince). At first it also meant crown prince in Mongolian. It was given only to descentans of Genghis Khan.
Qong Tayiji became sub-Khan since Altan Khan of the Tümed tümen installed Qong Tayiji as assistant Khad. This convention spread throughout the Mongols.
In 1630s the head of the Jüün Ghar was given the title of Baatur Qong Tayiji by the Dalai Lama. Since then Qong Tayiji was took over by the Jüün Ghar chiefs. As the Jüün Ghar got stronger, the title became higher. In the mid-18th century it went beyond Khan among the Oyirad because too many chiefs were appointed Khad by the Qing Dynasty.