Thalassius of Syria was a 5th-century monk and hermit who lived in the Byzantine Empire. Residing in a cave near the village of Pillima within the diocese of Cyrus, he lived a solitary life alongside his disciple, Limneus. He was a contemporary of the historian and bishop Theodoret, who visited him during his long period of asceticism. Known for his profound sanctity, he maintained a strict enclosure, permitting only his bishop to enter his stone dwelling while communicating with others through a small window.