Sima Tan (Chinese: 司馬談; Wade–Giles: Ssu-ma T'an, 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.