Phaenias of Eresus was a Greek philosopher of the 4th century BCE. He was from the city of Eresus on the island of Lesbos and was a student of Aristotle, making him a member of the early Peripatetic school. He was a contemporary and close associate of the more famous philosopher Theophrastus. The exact dates of his life are not recorded, but he was active in the later part of the century.

Phaenias was a prolific writer, though none of his works survive intact. They are known only through fragments quoted by later ancient authors. His writings demonstrate the wide-ranging interests of Aristotle's first followers. According to modern scholars, his known titles include works on history and politics, such as The Tyrants of Sicily; on philosophy and biography, like On the Socratics; on literary criticism, in On the Poets; and on natural science, notably a botanical treatise On Plants.

His historical significance lies in his role as an early disseminator of Aristotelian thought across multiple fields. Academics view his fragmentary corpus as a window into the encyclopedic scholarship of the early Peripatetic circle. He is particularly noted for his biographical and historical works, which were used as sources by later writers. While overshadowed by Theophrastus, Phaenias represents the broader intellectual activity within Aristotle's immediate school.