Jisun Si 季孫斯 (died 492 BCE), posthumous title Ji Huanzi 季桓子, was chief minister in the feudal state of Lu 魯 during the late Spring and Autumn period 春秋 (770-5th cent. BCE) and a member of the from the house of Jisun 季孫, one of the Three Huan families 三桓. He was the son of Ji Pingzi 季平子 and reigned betwen 501 and 492.
When Yang Hu 陽虎 tried usurping the throne in Lu, Jisun Si was the main force behind his downfall and lead the troops of Lu against his town Yanghu 陽關 (modern Ningyang 寧陽, Shangdong). After this incident Jisun Si took over regency for Duke Ai 魯哀公 (r. 495-467), while Confucius 孔子 functioned as counsellor-in-chief (xiang 相, zai 宰), but from 498 on the office fell to his disciple Zhong You 仲由 (better known as Zilu 子路).
During these years Jisun Si tried in vain to submit the family Mengsun 孟孫, another lateral branch of the ruling dynasty of Lu. When Jisun Si was presented a band of female singers by the duke of Qi 齊, Confucius, who was at that time Minister of Justice (sikou 司寇), left Lu because he did not want to accept such immoral behaviour. In 493 Jisun Si conducted a campaign against the statelet of Zhu 邾.