Sima Yuanxian (司馬元顯) (382 – before 3 February 402), courtesy name Langjun (郎君), formally Heir Apparent Zhong of Kuaiji (會稽忠世子), was briefly a regent of the Jin Dynasty (266–420), during the reign of his developmentally-disabled cousin Emperor An. Life Sima Yuanxian was the son of Sima Daozi the Prince of Kuaiji, the younger brother of Emperor Xiaowu and son of Emperor Jianwen, and he was born during the reign of his uncle Emperor Xiaowu. When Emperor Xiaowu was murdered by his concubine Honoured Lady Zhang in 396, he was succeeded by Emperor An, and Sima Daozi, as the emperor's uncle, became regent. Sima Daozi's trusted associates Wang Guobao () and Wang Xu (), because of their corrupt ways, quickly drew the ires of provincial officials Wang Gong () and Yin Zhongkan (), and in 397, Wang Gong and Yin started a rebellion, demanding that Wang Guobao and Wang Xu be killed. Sima Daozi, apprehensive of Wang Gong and Yin's power, executed Wang Guobao and Wang Xu, and Wang Gong and Yin withdrew. It was at this juncture that Sima Yuanxian, Sima Daozi's heir apparent, who was 15 but considered intelligent and capable, warned Sima Daozi that Wang Gong and Yin would one day again rebel. Sima Daozi, trusting his son, entrusted Sima Yuanxian with his personal guards. In 398, the provincial official Yu Kai (), unhappy that part of his domain was given to Wang Yu (), an official trusted by Sima Daozi, managed to persuade Wang Gong and Yin that the move was actually against them, and so Wang Gong and Yin started another rebellion. Sima Daozi, in fear, panicked and spent his days drinking, entrusting all important matters to Sima Yuanxian, who at the time appeared intelligent and resolute, reminding his followers of Emperor Ming. Soon, Sima Daozi persuaded Wang Gong's general Liu Laozhi () to betray him, and Wang Gong was captured and killed. Sima Daozi, under the suggestion of Huan Xiu (), then managed to cause dissension between Yin and his generals Huan Xuan and Yang Quanqi () by offering Huan and Yang key posts; although Huan and Yang nominally remained Yin's allies, Yin was forced to withdraw his troops, and from that point on no longer posed a major threat, as his domain had now been divided into three, with Huan and Yang each given a third. Sima Daozi, believing that his son had made