Cui Yin (崔胤) (854 – February 1, 904), courtesy name Chuixiu (垂休), nickname Zilang (緇郎), formally the Duke of Wei (魏公), was an official of the Chinese Tang dynasty, serving as a chancellor during the reign of Emperor Zhaozong. He was one of the controversial figures in the late Tang period, who ruthlessly tried to destroy the powerful eunuchs at court and whose actions in that regard had traditionally made him regarded as one of the persons causing the demise of the dynasty at the hands of the warlord Zhu Quanzhong the military governor of Xuanwu Circuit (宣武, headquartered in modern Kaifeng, Henan) (who would overthrow Tang and establish his own Later Liang).

Cui Yin was born in 854, during the reign of Emperor Xuānzong. He was from the prominent Cui clan of Qinghe (清河, in modern Xingtai, Hebei), was from the "Wushui branch " of the prominent Cui clan of Qinghe. Cui Yin's father Cui Shenyou was a chancellor during Emperor Xuānzong's reign. Cui Yin had at least one older brother, Cui Changxia (崔昌遐). (According to the biography of the powerful eunuch Qiu Shiliang in the New Book of Tang, Cui Yin developed a hatred for eunuchs early in his life due to Cui Shenyou's relating to him of an incident during the earlier reign of Emperor Xuānzong's nephew Emperor Wenzong, when Qiu tried to have Emperor Wenzong deposed.)

After Cui Yin passed the imperial examinations in the Jinshi (進士) class, he served on the staff of Wang Chongrong the military governor of Hezhong Circuit (河中, headquartered in modern Yuncheng, Shanxi). (Wang was military governor of Hezhong from 881 to Wang's death in 887, placing a timeframe on Cui's service under him.) Later, after Cui was recalled to the imperial government, he served successively as Kaogong Yuanwailang (考功員外郎) and then Libu Yuanwailang (吏部員外郎), both low-level positions at the ministry of civil service affairs (吏部, Libu). He was later promoted to a supervisory position at the ministry of civil service affairs (郎中, Langzhong). He was later made imperial attendant (給事中, Jishizhong) and then Zhongshu Sheren (中書舍人), a mid-level official at the legislative bureau of government (中書省, Zhongshu Sheng). During the Dashun era of Emperor Xuānzong's grandson Emperor Zhaozong (890–891), Cui successively served as deputy minister of defense (兵部侍郎, Bingbu Shilang) then deputy minister of civil service affairs (吏部侍郎, Libu Shilang). (It was said that his repeated promotions were a result of his close association with the chancellor Cui Zhaowei, who, while not a close relative, regarded him as a clansman.)

Thanks to his association with Cui Zhaowei, Cui Yin was promoted to be chancellor in 893 with the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事). As chancellor, he was described to be treacherous in his heart but appearing to be forgiving in his outward appearance. (It was also said that his uncle, the major general Cui Anqian, commented, "My father and brother had carefully built this household, but Zilang [(i.e., Cui Yin)] will destroy it!" although it was unclear whether Cui Anqian was still alive at this point.)

In 895, when Wang Chongrong's brother and successor Wang Chongying died at Hezhong (which had been renamed Huguo at this point), the soldiers supported Wang Chongrong's adoptive son and biological nephew Wang Ke to succeed him as the military governor, but Wang Chongying's son Wang Gong, then the military governor of neighboring Baoyi Circuit (保義, headquartered in modern Sanmenxia, Henan), also had designs on Huguo. Wang Ke was supported by his father-in-law Li Keyong the military governor of Hedong Circuit (河東, headquartered in modern Taiyuan, Shanxi), while Wang Gong was supported by his allies Li Maozhen the military governor of Fengxiang Circuit (鳳翔, headquartered in modern Baoji, Shaanxi), Wang Xingyu the military governor of Jingnan Circuit (靜難, headquartered in modern Xianyang, Shaanxi), and Han Jian the military governor of Zhenguo Circuit (鎮國, headquartered in modern Weinan, Shaanxi), and all these military governors submitted competing petitions to Emperor Zhaozong in support of Wang Ke or Wang Gong. Emperor Zhaozong initially approved Li Keyong's request and made Wang Ke the military governor of Huguo but, subsequently, trying to settle the dispute, named Cui the military governor of Huguo, still carrying the Tong Zhongshu Menxia Pingzhangshi designation as an honorary title. Wang Tuan was made a chancellor to replace him.

This failed to settle the dispute, however, and, at Cui Zhaowei's instigation (as Cui Zhaowei was allied with Li Maozhen and Wang Xingyu), the three military governors who supported Wang Gong marched on the imperial capital Chang'an, killing the chancellors Li Xi and Wei Zhaodu and forcing Emperor Zhaozong to accede to their demands. That, in turn, caused Li Keyong to launch his troops to attack them. Emperor Zhaozong, fearing that Li Maozhen and/or Wang Xingyu would seize him, fled into the Qinling Mountains. Cui Yin, along with chancellors Xu Yanruo and Wang Tuan, followed the emperor in flight. After Li Keyong defeated Wang Xingyu, who was then killed by his own subordinates, Li Maozhen and Han outwardly submitted to the emperor, and Emperor Zhaozong, after returning to Chang'an, ordered Li Keyong to end his campaign. Meanwhile, Cui was again made a chancellor, and also took the position of Zhongshu Shilang (中書侍郎), the deputy head of the legislative bureau. However, his ally, Cui Zhaowei, was exiled and then executed.

In 896, the fragile peace between the imperial government and Li Maozhen ended, and Li Maozhen again postured to attack Chang'an. Emperor Zhaozong and the imperial court initially decided to flee to Hedong, but due to the long distance involved and the urging of Han's, decided to head for Han's capital Hua Prefecture (華州) instead. Once the imperial court settled in at Hua Prefecture, however, Han inserted himself in all kinds of decision-making, and as one of his suggestions, Emperor Zhaozong, who was aware of the alliance between Cui Zhaowei and Cui Yin, decided to send Cui Yin away to serve as the military governor of Wu'an Circuit (武安, headquartered in modern Changsha, Hunan), still carrying the Tong Zhongshu Menxia Pingzhangshi designation as an honorary title; Cui was replaced with Lu Yi. Cui, however, secretly sought aid from Zhu Quanzhong the military governor of Xuanwu Circuit (宣武, headquartered in modern Kaifeng, Henan)—and suggested to Zhu that he could posture by preparing the palace at the eastern capital Luoyang and petitioning Emperor Zhaozong to move the capital to Luoyang. Zhu did so (along with his ally Zhang Quanyi the mayor of Henan Municipality (河南, i.e., the Luoyang region)) and also mobilized his troops; his petition also indicated his belief that Cui, as a faithful servant to the emperor, should not be sent away. Han, in fear of a potential Zhu attack, backed off, and Emperor Zhaozong subsequently kept Cui at court and made him a chancellor again, along with Cui Yuan. Cui Yin, displeased that Lu had replaced him, then falsely accused Lu of being an ally of Li Maozhen's; Lu was then exiled to the post of prefect of Xia Prefecture (硤州, in modern Yichang, Hubei).

Cui continued to serve as chancellor until 899, by which time Emperor Zhaozong had returned to Chang'an; Cui assumed just his regular posts of Zhongshu Shilang and minister of civil service affairs (吏部尚書, Libu Shangshu); Lu, who had been recalled by this point and was serving as the minister of defense (兵部尚書, Bingbu Shangshu), replaced him. (During this 896–899 term as chancellor, it was said that Cui secretly plotted with the emperor to slaughter the eunuchs. Wang Tuan, fearing the repercussions of such a plot, urged moderation, pointing out that a major confrontation between imperial officials and eunuchs would have uncertain results. When Cui was removed from the chancellor position in 899, he thus suspected Wang of being behind his removal and hated Wang from this point on; he also accused Wang of being in league with the powerful eunuch directors of palace communications Zhu Daobi (朱道弼) and Jing Wuxiu (景務脩).)