Sphaerus of Borysthenes was a Stoic philosopher of the 3rd century BCE. He was born in the Greek colony of Borysthenes on the Black Sea coast. He studied in Athens under Zeno of Citium, the founder of Stoicism, and later under Zeno’s successor, Cleanthes. Sphaerus spent part of his career as a philosophical advisor at the court of King Ptolemy IV in Alexandria and later with King Cleomenes III of Sparta. Ancient anecdotes describe his interactions with these rulers, illustrating his commitment to Stoic principles.

Although none of his writings survive complete, he was a prolific author. Known works, preserved only in fragments and titles cited by later writers, covered core Stoic subjects. These included ethics, physics, and logic, with titles such as On the Cosmos, On the Soul, On Eros, and On the Government of Sparta.

According to modern scholars, Sphaerus was a significant early figure in the Stoic school. He helped systematize and spread its doctrines in the generation after its founders. His role as an advisor to Hellenistic kings demonstrates how Stoic philosophy moved beyond academic circles into the realm of politics and power. His lost works were influential and cited by later ancient sources, contributing to the development of Stoic thought, particularly in psychology and ethics.