Cleanthes was a Greek philosopher who lived from approximately 331 to 232 BCE during the Hellenistic period. He was born in the city of Assos in Asia Minor and later moved to Athens. There, he worked as a manual laborer, famously carrying water to support himself while studying philosophy. He became a student of Zeno of Citium, the founder of Stoicism, and succeeded him as the second head of the Stoic school, leading it for over thirty years.

As a Stoic philosopher, Cleanthes wrote many works on logic, physics, ethics, and theology. However, nearly all of his writings are lost and survive only as fragments quoted by later authors. His most famous surviving work is the Hymn to Zeus, a poetic prayer that is the most complete text of his we have. Other known but fragmentary works include treatises On Pleasure, On the Gods, and On Time.

Cleanthes is historically important as a crucial link between the founder of Stoicism, Zeno, and its great systematizer, Chrysippus. According to modern scholars, he helped shape early Stoic doctrine, particularly in physics and theology. He emphasized a view of God as a rational, active force permeating the entire cosmos. His Hymn to Zeus is often interpreted as a key document showing how Stoicism combined traditional religious language with a philosophical belief in a single, rational world-order. While some of his specific ideas were later refined by Chrysippus, Cleanthes' perseverance and dedication were celebrated in antiquity and his role in establishing the Stoic school was fundamental.