Emperor Hui of Jin (simplified Chinese: 晋惠帝; traditional Chinese: 晉惠帝; pinyin: Jìn Huì Dì; Wade–Giles: Chin Hui-ti; 259 – January 8, 307), personal name Sima Zhong (司馬衷), courtesy name Zhengdu (正度), was the second emperor of the Western Jin dynasty. Emperor Hui was a developmentally disabled ruler, and throughout his reign, there was constant internecine fighting between regents, imperial princes (his uncles, half-brothers and cousins), and his wife Empress Jia Nanfeng for the right to control him (and therefore the imperial administration), causing great suffering for the people and greatly undermining the stability of the Western Jin dynasty, eventually leading to rebellions of the Five Barbarians that led to Jin's loss of northern and central China and the establishment of the competing Sixteen Kingdoms. He was briefly deposed by his granduncle Sima Lun, who usurped the throne himself, in February 301, but later that year was restored to the throne and continued to be the emperor until January 307, when he was poisoned, likely by his last regent Sima Yue.

Sima Zhong was born to Sima Yan and his wife Yang Yan in 259 AD, while Sima Yan was still the assistant to his father, the Cao Wei regent Sima Zhao. He was their second son, but after the early death of his older brother Sima Gui (司馬軌), he became the oldest surviving son. It is not known when his developmental disabilities became apparent, but, in any case, after Sima Zhao died in September 265, and Sima Yan subsequently forced the Cao Wei emperor Cao Huan to abdicate to him in February 266 (which ended Cao Wei's reign and started Jin's as Emperor Wu), he made the nine-year-old (by East Asian reckoning) Prince Zhong crown prince in 267 AD.

As Crown Prince Zhong grew in age, his developmental disabilities became clear to his parents and the imperial officials alike. He learned how to write and speak, but appeared to be unable to make logical decisions on his own at all. Once, when he heard frogs croaking, he asked, in all seriousness, "Do they croak because they want to, or because the government ordered them to?" Several times, officials reminded Emperor Wu of this, but Emperor Wu, not realizing the extent of Crown Prince Zhong's disability, resisted the implicit calls for him to be replaced. Indeed, because Emperor Wu was concerned that many officials were impressed with his talented younger brother, Sima You the Prince of Qi and might want Prince You to replace him instead, he eventually had Prince You sent to his principality, and Prince You died in anger in April 283.

On 2 April 272, at age 13, Crown Prince Zhong married Jia Chong's daughter Jia Nanfeng, who at 14 was two years older. Possibly biased histories describe Crown Princess Jia as violent and jealous, but she had her methods of controlling Crown Prince Zhong so that he both loved and feared her. They had four daughters during their marriage, but his only son, Sima Yu, was born to Consort Xie Jiu. Consort Xie was originally a concubine of Emperor Wu, but had been given to Crown Prince Zhong prior to his marriage to Crown Princess Jia, so that Consort Xie could teach him how to have sexual relations. Sima Yu was much favored by his grandfather Emperor Wu. Emperor Wu considered Prince Yu intelligent and very much like his own grandfather Sima Yi, and this played into his decision not to replace Crown Prince Zhong. However, other than Crown Princess Jia and Lady Xie, no other consort would bear Crown Prince Zhong a child, as Crown Princess Jia ordered each pregnant concubine murdered in fits of jealousy. (Emperor Wu, in anger, considered deposing Crown Princess Jia, but with the intercession of his second wife Empress Yang Zhi, he recalled Jia Chong's contributions to the establishment of the Jin dynasty, and decided to leave her in place.)

In 289, as Emperor Wu neared death, he considered whom to make the regent for Crown Prince Zhong. He considered both Empress Yang's father Yang Jun and his uncle Sima Liang the Prince of Ru'nan, the most respected of the imperial princes. As a result, Yang Jun became fearful of Sima Liang and had him posted to the key city of Xuchang (許昌, in modern Xuchang, Henan). By 290, Emperor Wu resolved to let Yang and Sima Liang both be regents, but after he wrote his will, the will was seized by Yang Jun, who instead had another will promulgated in which Yang alone was named regent. Emperor Wu died soon thereafter in May, and Crown Prince Zhong ascended the throne as Emperor Hui. Crown Princess Jia became empress, and Prince Yu became crown prince.

During his 16-year reign, Emperor Hui would come under the control of a number of regents and de facto princes, never being able to assert authority on his own. The rough succession order of the regents and de facto princes were:

Yang Jun quickly showed himself to be autocratic and incompetent, drawing the ire of many other nobles and officials. He tried to appease them by making many bestowments of titles and honors among them, but this only brought further contempt for his actions. He knew Emperor Hui's empress Jia Nanfeng to be strong-willed and treacherous, so he tried to put people loyal to him in charge of all the defense forces of the capital Luoyang, and also ordered that all edicts not only be signed by the emperor but also by Empress Dowager Yang before they could be promulgated.

Empress Jia, however, wanted to be involved in the government, and was angry that she was constantly rebuffed by Empress Dowager Yang and Yang Jun. She therefore conspired with the eunuch Dong Meng (董猛) and the generals Meng Guan (孟觀) and Li Zhao (李肇) against the Yangs. She tried to include Sima Liang into the conspiracy, but Sima Liang declined; instead, she persuaded Emperor Hui's brother, Sima Wei the Prince of Chu, to join her plan. In April 291, after Sima Wei returned to Luoyang from his defense post (Jing Province (荊州, modern Hubei and Hunan)) with his troops, a coup went into progress.