Hermarchus was a Greek philosopher from Mytilene who lived during the 4th and 3rd centuries BCE. He was a leading member of the Epicurean school, founded by Epicurus. Originally a student of rhetoric, Hermarchus became one of Epicurus's closest companions in the philosophical community known as the Garden in Athens. Upon Epicurus's death in 270 BCE, Hermarchus succeeded him as the head of the Epicurean school, a role he held for the rest of his life. Ancient sources show that Epicurus trusted him deeply, leaving the school's property in his care.

Hermarchus wrote several works to defend and explain Epicurean philosophy, but they survive only in fragments. His writings included letters and treatises. One of his most significant known works was a long polemical treatise Against Empedocles, which argued against that earlier philosopher's views on nature. He also wrote works with titles like On the Gods, On Discoveries, and On Mathematics, though their exact contents are now lost.

His primary historical importance was as the immediate successor to Epicurus. In this role, he was crucial for maintaining the stability and continuity of the Epicurean school in its early years. According to modern scholars, his leadership helped consolidate the community and defend its teachings against critics from other philosophical traditions. While his own original contributions are not clearly defined, his surviving fragments provide valuable evidence for early Epicurean debates and doctrines.