Achilles Tatius the Astronomer was a Greek author of the Roman imperial period, generally placed in the 2nd or 3rd century CE. This epithet distinguishes him from the novelist of the same name. Beyond this, his identity and biography are obscure. The Byzantine Suda mentions an Achilles Tatius who wrote on the sphere, but it is unclear if this refers to the astronomer.

His sole surviving work is On the Sphere, a short introductory treatise on spherical astronomy. It explains the celestial sphere, key circles like the horizon and ecliptic, the zodiac, and planetary motions, functioning as an elementary pedagogical text.

On the Sphere is a representative example of Greco-Roman elementary astronomical literature. Its significance lies in its role as a pedagogical text that transmitted foundational astronomical concepts. The treatise was later translated into Arabic and Latin, aiding the transmission of Greek astronomical knowledge to medieval scholars.