Wu Chengsi (; died July 22, 698), formally Prince Xuan of Wei (魏宣王), was a nephew of the Chinese sovereign Wu Zetian and an imperial prince of the Wu Zhou dynasty. He participated in her planning in taking the throne and had wanted to become crown prince after she claimed the throne in 690, but his attempts were repeatedly rebuffed, and after she showed her intent to eventually return the throne to her son Li Zhe by recalling Li Zhe from exile in 698, Wu Chengsi died in disappointment.
Background
It is not known when Wu Chengsi was born. His father Wu Yuanshuang (武元爽) was a half-brother of Wu Zetian—both had, as father, the early Tang Dynasty general Wu Shihuo (武士彠), but Wu Yuanshuang and his brother Wu Yuanqing (武元慶) were born of Wu Shihuo's first wife Lady Xiangli, while Wu Zetian and her two sisters were born of Wu Shihuo's second wife Lady Yang. After Wu Zetian became empress to Emperor Gaozong in 655, despite previous intrafamily unpleasantries—Wu Yuanqing and Wu Yuanshuang, as well as Wu Shihuo's nephews Wu Weiliang (武惟良) and Wu Huaiyun (武懷運) were often disrespectful of Lady Yang previously—Empress Wu's brothers and cousins were often promoted by Emperor Gaozong, with Wu Yuanshuang promoted to Shaofu Shaojian (少府少監), the deputy director of the office of palace supplies. However, sometime before 666, Empress Wu, angry that her brothers and cousins did not appreciate the promotions, had them all demoted, with Wu Yuanshuang demoted to be the prefect of Hao Prefecture (濠州, roughly modern Chuzhou, Anhui). Eventually, after further accusations, Wu Yuanshuang was exiled to Zhen Prefecture (振州, roughly modern Sanya, Hainan) and died there in exile. When Wu Yuanshuang was exiled, Wu Chengsi went to Zhen Prefecture with his father.
In 674, Empress Wu had Wu Chengsi recalled from exile to inherit the title of Duke of Zhou—which Emperor Gaozong had posthumously created Wu Shihuo. He was also made the minister of imperial clan affairs.
During Emperor Zhongzong's and Emperor Ruizong's first reigns
Emperor Gaozong died in 683, and was initially succeeded by his and Empress Wu's son Li Zhe the Crown Prince (as Emperor Zhongzong), but Empress Wu retained power as empress dowager and regent. In spring 684, after Emperor Zhongzong showed signs of independence, she deposed him and replaced him with another son, Li Dan the Prince of Yu, but wielded power even more tightly thereafter. Around this time, it appeared that Wu Chengsi became a close confidant of hers, and later that year, when the chancellor Liu Rengui, who was then in charge of the capital Chang'an (with Empress Dowager Wu, who disliked Chang'an and favored the eastern capital Luoyang, taking up permanent residence at Luoyang), offered to resign, it was Wu Chengsi that she sent to Chang'an to comfort Liu and to dissuade him from resigning. It was also Wu Chengsi who formally acted for her at Emperor Ruizong's enthronement. In summer 684, she made Wu Chengsi the minister of rites and gave him the designation Tong Zhongshu Menxia Sanpin (同中書門下三品), making him a chancellor de facto, although she removed him from that position three months later and made him the minister of ceremonies. Later that year, pursuant to a formal request of his, she posthumously created five generations of Wu ancestors princes or dukes and built a five-generation ancestral temple for them, second only to the imperial ancestral temple (which included seven generations). Meanwhile, he and a cousin, Wu Sansi (Wu Yuanqing's son), knowing that Empress Dowager Wu was contemplating taking the throne herself, advocated that two senior members of the Li imperial clan—Emperor Gaozong's uncles Li Yuanjia (李元嘉) the Prince of Han and Li Lingkui (李靈夔) the Prince of Lu—be killed, lest that they interfere with her plans. Empress Dowager Wu agreed, but did not immediately carry out the plan.
In 685, Empress Dowager Wu made Wu Chengsi chancellor again (this time with the modified designation Tong Fengge Luantai Sanpin (同鳳閣鸞臺三品), as she had changed many office names in the government structure) but removed him a month later.
In 686, fearful of the general Li Xiaoyi (李孝逸) -- a member of the imperial Li clan who had defeated the anti-Empress Dowager Wu rebellion led by Li Jingye the Duke of Ying in 684—Wu Chengsi made false accusations against him, and in response, Empress Dowager Wu demoted Li Xiaoyi. In 687, Wu Chengsi further accused Li Xiaoyi of having made comments that he would be emperor one day, and Empress Dowager Wu had Li Xiaoyi exiled to Dan Prefecture (儋州, modern Danzhou, Hainan), where he died in exile.