Emperor Jin Jianwendi 晉簡文帝 (r. 371-372), personal name Sima Yu 司馬昱, courtesy name Daowan 道萬, was an emperor of the Eastern Jin dynasty 東晉 (317-420). He was the youngest son of Emperor Yuan 晉元帝 (r. 317-322) and of Lady Zheng 鄭夫人.
Sima Yu started his career as Prince of Langya 瑯琊, later prince of Guiji 會稽. Under the rule of Emperor Mu 晉穆帝 (r. 344-361), he was General-in-chief supporting the army (fujun da jiangjun 撫軍大將軍) and overseer of the six imperial secretaries (lu shangshu liutiao shi 錄尚書六條事). In this function he took part in the government of eight reign-periods, from 366 even in the function of Counsellor-in-chief (chengxiang 丞相). Sima Yu was known for his profound words, his modesty, and his knowledge of literature and historiography.
In 371, Commander-in-chief (da sima 大司馬) Huan Wen 桓溫 forced emperor Sima Yi 司馬奕 (known as the "Deposed Emperor" Jin Feidi 晉廢帝, r. 365-371) to resign, and Empress Dowager Chu 褚太后 issued an edict giving the throne to Sima Yu. He accepted and chose the reign motto Xian'an 咸安 "All-pervading peace".
During Sima Yu's reign, Huan Wen acted as regent. The emperor accepted all suggestions of the regent, for fear he might be dethroned, too. A year later, the emperor's son Sima Yao 司馬曜 was given the title of Heir Apparent (huang taizi 皇太子), early enough, because Sima Yu died shortly after. In his last will, he ordered Huan Wen to reign for the young ruler (known as Emperor Xiaowu 晉孝武帝, r. 372-396). Sima Yu was buried in Mound Gaoping 高平陵 and was given the posthumous title Jianwen 簡文 "Austere-Cultured" and the dynastic title Taizong 晉太宗.