Wu Sansi (died August 7, 707), known posthumously as Prince Xuan of Liang (梁宣王), was a Chinese prince and politician of the Tang and Wu Zhou dynasties. Wu Sansi served as a chancellor and imperial prince during the reign of his aunt, Empress Wu Zetian of Zhou, and was again a powerful chancellor during the second reign of Empress Wu's son, Emperor Zhongzong of Tang. He was aided in navigating the fraught period that followed Emperor Zhongzong's restoration by successive affairs with the influential consort Shangguan Wan'er and the powerful Empress Wei. Although he amassed significant authority and enjoyed the trust of Emperor Zhongzong, he was killed during an unsuccessful rebellion by the crown prince Li Chongjun in 707.

Wu Sansi was inherently ambitious, talented, and politically ruthless. Even during the reign of Wu Zetian, he had aspired to become crown prince. With the return of Emperor Zhongzong in the coup of 705, which forced Wu Zetian to abdicate the throne, the 15-year-old Wu Zhou dynasty (690-705) founded by her came to an end, and at the same time a nearly 50-year period (655-705) of her political dominance also concluded. However, her legacy remained enduring. Wu Sansi soon found a place within the factional struggles of the new reign and became a trusted and powerful advisor in the emperor’s inner circle, to the extent that the emperor consulted him on every matter, whether minor or major. He also formed an alliance with Emperor Zhongzong’s wife, Empress Wei. After forming an alliance with Empress Wei, who had boundless ambition, it was natural that they could not tolerate Zhang Jianzhi and his faction—Cui Xuanwei, Jing Hui, Huan Yanfan, and Yuan Shuji—who held power in the court. Accordingly, both from within and outside, they repeatedly slandered them in the presence of Emperor Zhongzong and tried to harm them.

Emperor Zhongzong, who was weak-willed, naturally timid, easily influenced by those close to him, and, more importantly, unable to distinguish right from wrong, readily believed these accusations. This created the conditions for the downfall of Zhang Jianzhi, Cui Xuanwei, Jing Hui, Huan Yanfan, and Yuan Shuji. Emperor Zhongzong ruled relying on Empress Wei and Wu Sansi, giving them—and through them, his daughter Princess Anle—free rein to do whatever they wanted, while remaining indifferent to rumors, accusations, or evidence against them. At that time, Wu Sansi was appointed Chancellor (宰相) and was responsible for policy-making, legislation, and the ranking of court officials, advancing the interests of his own faction within court politics. Empress Wei, alongside the emperor, would attend the court audience hall and, from behind a curtain, supervise court affairs and intervene in governance. Shangguan Wan'er, formerly Wu Zetian’s secretary, was now a concubine of Emperor Zhongzong with the title Zhaorong (昭容). She simultaneously held responsibility for drafting imperial edicts (掌管宫中制诰) and served as Director of the Imperial Archives (蘭台令史). She was in collusion with Wu Sansi and also had an illicit relationship with him; therefore, she aligned closely with Empress Wei’s faction and provided them with significant support.

Wu Sansi, after successfully eliminating his political opponents and gaining substantial power, reinstated some of the policies and practices from the era of Wu Zetian. He colluded with several officials—Zong Chuke, Minister of War; Zong Jinqing, Minister of Works; Ji Chu, Minister of the Treasury; and Gan Yuanjian, Minister of Ceremonial Affairs—appointing them as his close associates. These four became known at court as the “Four Allies of Wu Sansi.” At the same time, he also designated five other officials as his “eyes and ears”: Zhu Lizhen, Deputy Chief Inspector; Ran Zuiyong, Assistant Minister of the Court; Li Quan, Imperial Tutor (a nephew of the poet Song Zhiyun); Song Zhishun, Assistant Director of Imperial Banquets (younger brother of Song Zhiyun); and Yao Shaozhi, Court Inspector. Both inside and outside the court, these five were referred to as the “Five Dogs of Wu Sansi.”

After eliminating the five coup leaders, known as the “Five Princes,” Wu Sansi and Empress Wei turned their attention to removing Crown Prince Li Chongjun in order to replace him with Princess Anle. However, Li Chongjun, with the support of opponents of the Wu-Wei faction, staged a coup in 707, which resulted in the deaths of Wu Sansi and his son Wu Chongxun. Yet, the coup failed in its primary goal of seizing the palace and killing Empress Wei and Princess Anle, and Li Chongjun himself was killed.

It is not known when Wu Sansi was born. His father Wu Yuanqing (武元慶) was a half-brother of Wu Zetian—both had, as father, the early Tang dynasty general Wu Shihuo (武士彠), but Wu Yuanqing and his brother Wu Yuanshuang (武元爽) 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 Yuanqing promoted to Zongzheng Shaoqing (宗正少卿), the deputy minister of imperial clan affairs. As the years passed, more and more power passed to Empress Wu, who until 665 had complete control of power, which was the beginning of her ascension to the imperial throne, which made his family increasingly in the highest honorable in among the powerful families of the empire. However, sometime before 666, Empress Wu, angry that her brothers and cousins did not appreciate the promotions, had them all demoted, with Wu Yuanqing demoted to be the prefect of Long Prefecture (龍州, roughly modern Mianyang, Sichuan). After he arrived at Long Prefecture, he, in fear that Empress Wu had further retaliation in the works, died. It is not completely clear, but it was likely that Wu Sansi went to Long Prefecture with his father Wu Yuanqing. In 675, at the behest of Empress Wu, Wu's family was pardoned and Wu Sansi was hailed as the empress's nephew, and he was made a commanding general of the imperial guards.

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. Sometime during her regency, Wu Sansi became minister of defense (夏官尚書, Xiaguan Shangshu). Both he and his cousin Wu Chengsi (Wu Yuanshuang's son) advised Empress Dowager Wu to find excuses to kill two senior members of the imperial Li clan—Emperor Gaozong's uncles Li Yuanjia (李元嘉) the Prince of Han and Li Lingkui (李靈夔) the Prince of Lu due to their senior status. (Eventually, after two other princes—Emperor Gaozong's brother Li Zhen the Prince of Yue and Li Zhen's son Li Chong the Prince of Langye unsuccessfully rose against Empress Dowager Wu in 688, Empress Dowager Wu took the opportunity to also force Li Yuanjia and Li Lingkui, as well as many other members of the Li clan, to commit suicide or to execute them.) Wu Sansi and Wu Chengsi were also said to be often advocating that Empress Dowager Wu take over the throne herself as "emperor."

In 690, Empress Dowager Wu had Emperor Ruizong yield the throne to her, and she took the throne as "emperor," establishing a new Zhou dynasty and interrupting Tang. She created a number of her Wu clan relatives imperial princes, and Wu Sansi was created the Prince of Liang and made the minister of civil service affairs (天官尚書, Tianguan Shangshu). He did not appear to have as much power as Wu Chengsi (who became chancellor), but was nevertheless honored. For example, in 693, when Wu Zetian made sacrifices to heaven and earth, she offered the sacrifices herself first, followed by Wu Chengsi, and then Wu Sansi. It was said that he had done some studies in literature and history and was good at flattery. As the years went by, he often flattered Wu Zetian's successive lovers Huaiyi, Zhang Yizhi, and Zhang Changzong.