Sima Yong (司馬顒)[3] (before 274 - c.late January 307[4]), courtesy name Wenzai (文載), was a Jin dynasty imperial prince and briefly a regent for Emperor Hui.
Though a distant cousin of Emperor Hui, Sima Yong was garrisoned at the important city of Chang'an to guard the Guanzhong region.
Sima Yong was defeated the following year and lost the emperor, but he was able to recapture Chang'an and held out in the city before he was assassinated in 307.
On 19 September 291,[7] during the reign of Emperor Hui, Sima Yong was sent to garrison Yecheng.
In February or March 299,[8] he was put in charge of defending the important city of Chang'an—a post which Emperor Wu had left instructions that only someone who was closely related to the emperor should be allowed to take, but which the high level officials found appropriate for Sima Yong due to his reputation.
He put people he trusted in charge of the defenses of Luoyang, while remotely controlling the government from Yecheng.
Eventually, the officials in Luoyang grew tired of the situation, and they rose under Sima Yue's command in summer 304.
Sima Ying was forced to flee to Luoyang with Emperor Hui, now without troops to support him.
By summer 306, Sima Yong was forced to abandon both Chang'an and Emperor Hui, and Sima Yue's forces welcomed Emperor Hui back to Luoyang on 28 June.
Sima Yong agreed to accept it and started to travel back to Luoyang.
However, when he got to Xin'an (新安, near Luoyang), he was intercepted by Liang Chen (梁臣), a general under Sima Mo (司馬模; Prince of Nanyang and Sima Yue's younger brother) and strangled to death.