Yang Zaisi (楊再思) (died 709), formally Duke Gong of Zheng (鄭恭公), was a Chinese politician in the Tang dynasty, serving several times as chancellor during the reigns of Wu Zetian and her son Emperor Zhongzong.
It is not known when Yang Zaisi was born, but it is known that his family was from Zheng Prefecture (鄭州, roughly modern Zhengzhou, Henan).
He passed the imperial examination when he was young and was made the sheriff of Xuanwu County (玄武, in modern Deyang, Sichuan).
In 697, during a rebellion by Khitan's khan Sun Wanrong, Eastern Tujue's khan Ashina Mochuo, who had been at times ally and at times enemy of Wu Zetian's Zhou dynasty, indicated that he was willing to be aligned with Zhou against Khitan and made a number of requests for return of Tujue people who had surrendered to Zhou, various treasures, and food supplies, which, after Yang and fellow chancellor Yao Shu argued were necessary to maintain an alliance with Eastern Tujue, Wu Zetian agreed to give Ashina Mochuo, allowing him to become even stronger than before.
In 699, Yang was removed from his posts and made imperial censor reviewing central government affairs (左臺大夫, Zuotai Daifu), no longer chancellor.
Later in 704, he was serving as the official in charge of then-capital Luoyang while Wu Zetian was at Chang'an, when he was made Neishi (內史), the head of the legislative bureau (鳳閣, Fengge) and a post considered one for a chancellor.
Yang, taking the joke in stride, immediately made a hat, put it on, and did a Goguryeo-style dance, causing the guests to laugh.
During his years serving under Wu Zetian, he also received progressively greater noble titles, from Baron of Hongnong eventually to Duke of Zheng.
Four months later, he was made Shizhong (侍中), the head of the examination bureau (now changed to Menxia Sheng (門下省)) and still chancellor.
In spring 708, Yang was made You Pushe (右僕射), one of the heads of the executive bureau (尚書省, Shangshu Sheng), and still chancellor with the designation Tong Zhongshu Menxia Sanpin.