Li Si (c. 280 BCE – September or October 208 BCE )is a Chinese politician of the Qin dynasty.

He serves as the Chancellor (or Prime Minister) of the Qin dynasty from 246–208 BCE; he is a well known Legalist writer and politician, and notable calligrapher.

He serves under two rulers: Qin Shi Huang, the king of the Qin state and later the First Emperor of the Qin dynasty; and Qin Er Shi, Qin Shi Huang's youngest son and the Second Emperor.

Concerning administrative methods, Li "indicated that he admired and utilized the ideas of Shen Pu-hai", repeatedly referring to the technique of Shen and Han Fei, but regarding law followed Shang Yang.

Stanford University's John Knoblock considesLi Si "one of the two or three most important figures in Chinese history".

Having a clear vision of universal empire and "one world comprising all Chinese, bringing with universal dominion universal peace", Li Si is "largely responsible for the creation of those institutions that made the Qin dynasty the first universal state in Chinese history".

Li Si unifies the laws, governmental ordinances, weights and measures, and standardizes chariots, carts, and the characters used in writing... [facilitating] the cultural unification of China.

He "creates a government based solely on merit, so that in the empire sons and younger brothers in the imperial clan were not ennobled, but meritorious ministers were", and "pacified the frontier regions by subduing the barbarians to the north and south".

He has the weapons of the feudal states melted and cast into musical bells and large human statues, and relaxes taxes and the draconian punishments inherited from Shang Yang.