Cleostratus of Tenedos was an early Greek astronomer from the island of Tenedos, active in the late 6th or early 5th century BCE. He is credited with introducing the constellations Aries and Sagittarius into Greek astronomy and with developing the eight-year lunisolar calendrical cycle known as the octaeteris, a system later refined by Meton and Euctemon. Ancient sources suggest his work may reflect Babylonian influence, positioning him in a transitional phase between mythological cosmologies and systematic astronomy.

His lost works include the Astronomia, on constellations and celestial phenomena, and the Phainomena, a title commonly used for works describing stellar appearances and weather signs.

Cleostratus helped standardize the Greek celestial map, providing a foundation for later astronomical tradition. His practical work on the octaeteris represents a significant early effort to create a reliable calendar based on astronomical observation.