Sima Tan (, c. 165 BC – 110 BC) was an early Chinese historian who worked under the Western Han. He studied astronomy with Tang Du, the I Ching under Yang He, and Daoism under Master Huang. He held the position of Court Astrologer (太史令) between 140-110 BC. While Sima Tan had begun the Records of the Grand Historian (Shiji), he died before it was finished. It was left to his son, Sima Qian, to complete. The year of Sima Tan's death is the year of the great imperial sacrifice fengshan by Han Wudi, for which the emperor appointed the fangshi, leaving Sima behind and thus probably causing him much frustration.