Sima Jiong (Chinese: 司馬冏) (before 283 – 27 January 303), courtesy name Jingzhi (景治), formally Prince Wumin of Qi (齊武閔王), was an imperial prince of the Jin dynasty of China. He briefly served as Emperor Hui's regent after overthrowing the usurper Sima Lun in May 301. He was the fourth of the eight princes commonly associated with the War of the Eight Princes.

The son of Sima You, he played a role in Sima Lun's coup against Empress Jia and her family, but was later transferred away from the capital, Luoyang to guard the city of Xuchang. When Sima Lun usurped the throne from Emperor Hui, Sima Jiong raised an army and led the coalition of the "Three Princes" (三王) to restore Emperor Hui to the throne. He was appointed the new regent of Emperor Hui after Sima Lun's defeat, but his arrogant and wasteful personality made him deeply unpopular within the court. Soon, his former allies the Prince of Hejian, Sima Yong and the Prince of Chengdu, Sima Ying accused him of treason and threatened Luoyang with their forces. Believing that the Prince of Changsha, Sima Ai was in league with the two from inside the city, Sima Jiong wanted to execute him first, but Ai retaliated and killed him after a few days of fighting.

Sima Jiong was a son of Sima You, posthumously known as Prince Xian of Qi; You was the younger brother of Jin Dynasty's founder Emperor Wu, making Jiong Emperor Hui's cousin. Jiong was also a paternal grandson of Sima Zhao and Wang Yuanji, and a maternal grandson of Jia Chong and his first wife Li Wan. Sima You's abilities were so highly regarded by both his father Sima Zhao and the officials that, at times, he was considered as the proper heir, first for Sima Zhao, then for Emperor Wu. After Emperor Wu exiled him from the capital Luoyang in January 283, he died in April that year. Sima Jiong, as the oldest son of his wife Consort Jia Bao, inherited his post, and complained to Emperor Wu about doctors' misdiagnosis.

As an imperial prince, Sima Jiong had a sizable number of troops under his command. When Sima Lun overthrew Emperor Hui's domineering wife Empress Jia Nanfeng in May 300 after she murdered Emperor Hui's crown prince Sima Yu in April, Sima Jiong was a participant of the coup — despite Empress Jia being his half-maternal-aunt, given that his mother had a feud with Empress Jia. But Sima Jiong did not receive great rewards afterwards. He therefore resented Sima Lun. Sima Lun, seeing his resentment, tried to appease him by giving him an important military command—of Xuchang (許昌, in modern Xuchang, Henan).

After Sima Lun usurped the throne in February 301, he became concerned about Sima Jiong, Emperor Hui's half-brother Sima Ying the Prince of Chengdu and distant cousin Sima Yong the Prince of Hejian, each of whom had strong independent commands. He sent his close associates to serve as their assistants. Prince Jiong refused and declared a rebellion to restore Emperor Hui. Prince Ying, Sima Ai the Prince of Changshan (Emperor Hui's half-brother), and Sima Xin (司馬歆) the Duke of Xinye (a son of Sima Jun, a granduncle of Emperor Hui) all declared support for Prince Jiong. Prince Yong initially sent his general Zhang Fang (張方) with intent to support Sima Lun, but then heard that Princes Jiong and Ying had great forces, and so declared for the rebels instead. Sima Jiong's forces initially were stuck in a stalemate against Sima Lun's. But after Sima Ying's forces scored a major victory against another contingent of Sima Lun's troops, Sima Lun's troops collapsed, and Sima Jiong's and Sima Ying's forces approached Luoyang. Sima Lun was captured by officials in Luoyang who declared for the rebellion as well, and forced to issue an edict returning the throne to Emperor Hui. He was then forced to commit suicide in early June. Sun Xiu and other associates of Sima Lun had been executed earlier, in late May.

Some thought that a power balance that Emperor Wu had hoped for at his death might be restored; on August 11, Princes Jiong and Ying were each given regent titles (and awarded the nine bestowments, in one rare case where the nine bestowments were not signs of an impending usurpation, although Prince Ying declined the bestowments, and many talented officials were promoted into important positions. However, the Princes Jiong and Ying were actually apprehensive of each other's power, and Prince Ying decided to yield the central government regency to Prince Jiong at the time and return to his defense post at Yecheng (鄴城, in modern Handan, Hebei).

After Sima Jiong defeated Sima Lun, while other clansmen of the Sima family and court officials sent oxen and wine as gifts to Sima Jiong, Jiong's granduncle Sima Gan offered him 100 cash. At their meeting, Sima Gan said, "The Prince of Zhao (Sima Lun) caused a rebellion and you managed to put it down; that is your credit. Now, I'm congratulating you using these 100 cash. Even so, it is very difficult to remain at a high position; you must be careful and cautious."

Sima Jiong became arrogant based on his accomplishments. He had his sons created princes, and ran the matters of the central government from his mansion, rarely visiting the emperor or attending the imperial meetings. He enlarged his mansion to be as large as the palace, and he entrusted matters to people who were close to him, and would not change his ways even when some of his more honest associates tried to change his behavior. When Emperor Hui's grandson Sima Shang (司馬尚; son of Sima Yu) died in childhood, leaving Emperor Hui without male descendants by May 302, Sima Ying was considered the appropriate successor, but Sima Jiong chose to bypass him by recommending the seven-year-old Sima Qin (司馬覃) the Prince of Qinghe (Emperor Hui's nephew and the son of his half-brother Sima Xia (司馬遐)) as the crown prince, with intent to easily control the young Crown Prince Qin.