Xi Zezong (June 6, 1927, Yuanqu, Shanxi – December 27, 2008, Beijing) was a Chinese astronomical historian. He is best known for finding in ancient Chinese history a reference to Ganymede being visible to the human eye by ancient astronomer Gan De, before it was officially recognised by the West. He was a member of the Chinese Academy of Sciences, and an Awardee of the Astronomy Prize.
Xi graduated from the Astronomy Department of Zhongshan University in Zhongshan, and began a career in the Chinese Academy of Sciences in which he spent all of his life as a professor. He conducted research and supervised students beginning in 1981 at the university's Institute for History of Natural Science; he also served as the director of this Institute from 1983 to 1988. His main focus was on the study of historical supernovae, and he defined seven criteria with which to identify novae and two criteria to distinguish the novae and supernovae. He studied ancient novae recorded in Chinese, Korean and Japanese annals and in 1955 rewrote this into a modern-day catalogue, analysing their significance to radio astronomy. This was reprinted in 1965 in the United States and is considered an important reference work to astronomical scholars worldwide.
In the 1970s Xi undertook research for the unearthed astronomical materials at Mawangdui, and came across a valuable ancient chart which illustrated 29 types of comets. Xi was elected member the International Academy of History of Science in 1993, and of International Eurasian Academy of Sciences in 1995.
In 1996 he began studying celestial phenomenon records in an effort to accurately pinpoint dates for historical events, publishing a chronological table in November 2000. In 1997, a main-belt asteroid was named after him.
Xu was president of Chinese Society of History of Science and Technology, and was an academic advisor to the Research Center of Ancient Civilization, and was affiliated with many of China's top universities as a professor.