Theodosius of Bithynia was a Greek astronomer and mathematician of the Hellenistic period, active in the 2nd or 1st century BCE. His ethnonym indicates an origin from Bithynia in northwest Asia Minor. Beyond this geographical association and his profession, no biographical details are recorded.

He is known for three surviving treatises written in technical Koine Greek. His most influential work is the Sphaerica, a three-book compilation on spherical geometry essential for astronomy. The other two are On Habitations, describing celestial phenomena visible from different latitudes, and On Days and Nights, analyzing the varying length of day and night.

Theodosius's primary significance lies in his systematic exposition of spherical geometry. His Sphaerica became a standard textbook, replacing the earlier work by Autolycus of Pitane and was studied for centuries, with commentaries by Pappus and translations into Arabic and Latin. His works collectively formed a core part of the later astronomical curriculum, securing his role as a key transmitter of Hellenistic mathematical astronomy.