Yi Ji (fl. 200s–221), courtesy name Jibo, was a Chinese diplomat and politician serving in the state of Shu Han during the Three Kingdoms period of China. He previously served under the warlord Liu Biao in the late Eastern Han dynasty before coming to serve Liu Bei, the founding emperor of Shu Han. Yi Ji was known as an excellent debater and helped write the Shu Ke (蜀科), the legal code of Shu, along with Zhuge Liang, Fa Zheng, Li Yan and Liu Ba.

Yi Ji was born in Shanyang Commandery (山陽郡), which covered roughly the area around present-day Juye County, Shandong. At a young age, he became a subordinate of fellow Shanyang man Liu Biao the Governor of Jing Province (covering present-day Hubei and Hunan) at a time when Liu Biao held the rank of General Who Guards the South (鎮南將軍), a position given by Li Jue's junta, placing the start of service at 192 at earliest. In the early 200s, when the warlord Liu Bei sought shelter under Liu Biao after his rival Cao Cao led troops to attack Runan. Liu Biao offered to let him stay in Xinye County in northern Jing Province. Yi Ji met Liu Bei several times around the time and became an acquaintance of Liu Bei.

After Liu Biao's death in 208, his younger son Liu Cong, who succeeded him, surrendered to Cao Cao when Cao led his forces on a campaign in southern China. Yi Ji joined Liu Bei on a journey southward towards Xiakou (夏口; present-day Wuhan, Hubei), which was guarded by Liu Biao's elder son Liu Qi and was independent of Cao Cao's control. Yi Ji became an official under Liu Bei since then and accompanied him in Yi province. In July 214, after Liu Bei seized control of Yi Province (covering present-day Sichuan and Chongqing) from its governor Liu Zhang, Yi Ji followed Liu Bei to Chengdu, the capital of Yi Province. Liu Bei, who held the nominal appointment of General of the Left (左將軍) under the Han central government then, appointed Yi Ji as an Assistant Officer of the Household (從事中郎) under him. Liu Bei's treatment of Yi Ji was second to that of Jian Yong, Sun Qian and others.

Liu Bei later sent Yi Ji as a messenger to Jiangdong (or Wu) to meet the warlord Sun Quan, Liu's ally in their war against Cao Cao. Sun Quan had heard of Yi Ji's debating skills before and he wanted to test Yi. When Yi Ji met Sun Quan, he knelt down to pay his respects. Sun Quan asked: "Are you tired of serving an incompetent lord?" Yi Ji immediately replied: "I just need to kneel down and pay my respects and then stand up. This isn't tiring to me." Yi Ji's quick thinking was always as such. Sun Quan was deeply impressed by him.

After the fall of the Han dynasty in 220, Yi Ji served in the state of Shu Han – founded by Liu Bei in 221 – during the Three Kingdoms period. He was promoted to the position of General of Illustrious Writing (昭文將軍). He drafted the laws of Shu with Zhuge Liang, Fa Zheng, Liu Ba and Li Yan. The records of his life ends with his work on the Shu Ke, the laws of Shu.

Chen Shou, who wrote Yi Ji's biography, commented as follows: "Mi Zhu, Sun Qian, Jian Yong and Yi Ji were refined and cultured persons whose ideas were widely circulated. They were well known for their good observation of the proprieties."

First appears in the conquest of Yi, sent by Yi Ji to persuade the disaffected Zhang Yi to surrender Ziwu, where he had been held off Zhao Yun and then Zhang Fei. After the fall of Chengdu, Ma Chao arrived with thirty thousand troops and after wounding Wei Yan with an arrow, Yi Ji was sent by Liu Bei to persuade Ma Chao to surrender. Assigned the key defensive point of Yangping Pass, Liu Bei heads there when Cao Cao pushes on from conquest of Hanzhong and decides to push on into Liu Bei's new lands. Ma Chao is assigned with Yu Ji to help protect Yangping but Ma Chao is drunk and the pass falls to Zhang Liao. Yi Ji flees with less than a hundred horsemen back to Liu Bei after heavy pursuit by Cao Cao during the night.

After Liu Bei's death, reports of Yong Kai's revolt with the three southern cities and alliance with King Meng Huo though Zhuge Liang had already foreseen this would happen. During Zhuge Liang's third northern campaign when Zhuge Liang and Sima Yi are stalemated near Jieting, dismisses rumours at court that Zhuge Liang has rebelled and suggests Emperor Liu Shan summon Zhuge Liang to see if Zhuge Liang will come, proving Zhuge Liang's loyalty. As Yu Ji expected, Zhuge Liang obeyed the summons and blamed the matter as a trick by Sima Yi. Upon Sun Quan's death, Yi Ji sent with thousand strings worth of gold and gems to offer condolences and ensure Eastern Wu didn't cause trouble.