This week's episode of In Our Time on Epicureanism suggested that Epicurus and many of his followers could be valid examples of "atheists" in "polytheistic" ancient Greece.
According to one guest Epicurus' atomic theory (which build on Democritus') insists that each and everything, and thus including gods, is made of material atoms. Now whether this supports or rejects a belief in the existence of deities, as in atheism, is perhaps a question of interpretation. (Lucretius' On the Nature of Things certainly contains many references to "gods".)
However, one of the books on the accompanying reading list (Catherine Wilson: Epicureanism at the Origins of Modernity) includes the following description in its summary:
The target of sustained and trenchant philosophical criticism by
Cicero, and of opprobrium by the Christian Fathers of the early
Church, for its unflinching commitment to the absence of divine
supervision and the finitude of life, the Epicurean philosophy
surfaced again in the period of the Scientific Revolution, when it
displaced scholastic Aristotelianism.
This would suggest that there effectively was at least element of atheism (or deism) in Epicureanism.