Greek statesman of the Hellenistic Period, a skilled diplomatist and guerrilla fighter who for many years was the leading spirit of the Achaean League.

After liberating Sicyon in 251, he established a democracy there and united it with the Achaean League for defense against Macedonia. As general of the league (a post he normally held each alternate year after 245), he captured Acrocorinth (243), defeated the Aetolians at Pellene (241), and pursued a policy of establishing democracies in the Peloponnese. With Aetolia as ally from 239, Sicyon repeatedly attacked Athens and Argos. Aratus brought Megalopolis (235) and Argos (229) into the league and helped liberate Athens from Macedonian rule (229). The hostility of Sparta, however, threatened these gains.

After being defeated twice by the Spartans under Cleomenes III, Aratus' League was saved by timely support from Antigonus Doson (king of Macedonia, 227–221) in 224. A combined force of Achaeans and Macedonians defeated and dethroned Cleomenes in 222. On the accession of Philip V of Macedonia in 221, Aratus countered Aetolian aggression by obtaining the assistance of the Hellenic League. The resulting war ended in 217, and Aratus then began to resist Philip's anti-Roman policy and his interference in Messene. Although Philip was popularly believed to have had Aratus murdered, the Greek leader probably died of tuberculosis. His memoirs, no longer extant, provided an important source for Polybius' History.