Emperor Jin Mudi 晉穆帝 (r. 344-361), personal name Sima Dan 司馬聃, courtesy name Pengzi 彭子, was an emperor of the Eastern Jin dynasty. He was a son of Emperor Kang 晉康帝 (r. 342-344) and Empress Chu 褚皇后. His father fell ill when Sima Dan was just 2 sui old, and he was therefore appointed Heir Apparent (huang taizi 皇太子), so he could take over the throne immediately when Emperor Kang passed away. Regency was taken over by Empress Dowager Chu 褚太后, yet with the support of some princes, particularly Simy Yu 司馬昱, the Prince of Guiji 會稽 and eventual Emperor Jianwen 晉簡文帝 (r. 371-372).
At that time north China was in great turmoil because various Chinese and non-Chinese warlords fought against each other. This seemed to be a chance to reconquer northern China. In 347, the regional inspector (cishi 刺史) of the province of Jingzhou 荊州, Huan Wen 桓溫 (312-373), was sent out to attack Chengdu 成都 in the Sichuan Basin. He succeeded and destroyed the Cheng-Han empire 成漢 (304-347). The victory did not only bring Huan Wen fame and glory, but made him the most powerful man of the Jin empire.
A further military campaign was carried out in 354 and was directed against the Former Qin empire 前秦 (351-394), whose capital was Chang'an 長安 (today's Xi'an 西安, Shaanxi). Yet shortages in the supply caused the troops to return. When Sima Dan reached the age of 14 sui, he formally held his first court audience. At that occasion, his spouse, Ms He 何氏, was made empress, and the reign motto Shengping 升平 "Rise of Justice" was chosen. Moreover, a representative of the imperial house was sent to Luoyang 洛陽 (today in Henan), the old capital of the Western Jin dynasty 西晉 (265-316), to pay deference at the tombs of the former emperors.
Sima Dan died in 361. He was buried in Mound Yongping 永平陵 close to the imperial capital in Jiankang 建康 (today's Nanjing 南京, Jiangsu). His posthumous title was Emperor Mu 穆 "The Solemn", and his dynastic title Emperor Xiaozong 晉孝宗.
His successor was his cousin Sima Pi 司馬丕, known as Emperor Ai 晉哀帝 (r. 361-365).