Religious practices in ancient Greece encompassed a collection of beliefs, rituals, and mythology, in the form of both popular public religion and cult practices. The application of the modern concept of "religion" to ancient cultures has been questioned as anachronistic. The ancient Greeks did not have a word for 'religion' in the modern sense. Likewise, no Greek writer is known to have classified either the gods or the cult practices into separate 'religions'. Instead, for example, Herodotus speaks of the Hellenes as having "common shrines of the gods and sacrifices, and the same kinds of customs".

Most ancient Greeks recognized the twelve major Olympian gods and goddesses—Zeus, Hera, Poseidon, Demeter, Athena, Ares, Aphrodite, Apollo, Artemis, Hephaestus, Hermes, and either Hestia or Dionysus—although philosophies such as Stoicism and some forms of Platonism used language that seems to assume a single transcendent deity. The worship of these deities, and several others, was found across the Greek world, though they often have different epithets that distinguished aspects of the deity, and often reflect the absorption of other local deities into the pan-Hellenic scheme.

The religious practices of the Greeks extended beyond mainland Greece, to the islands and coasts of Ionia in Asia Minor, to Magna Graecia (Sicily and southern Italy), and to scattered Greek colonies in the Western Mediterranean, such as Massalia (Marseille). Early Italian religions such as the Etruscan religion were influenced by Greek religion and subsequently influenced much of the ancient Roman religion.

"There was no centralization of authority over Greek religious practices and beliefs; change was regulated only at the civic level. Thus, the phenomenon we are studying is not in fact an organized "religion". Instead we might think of the beliefs and practices of Greeks in relation to the gods as a group of closely related "religious dialects" that resembled each other far more than they did those of non-Greeks."

Ancient Greek theology was polytheistic, based on the assumption that there were many gods and goddesses, as well as a range of lesser supernatural beings of various types. There was a hierarchy of deities, with Zeus, the king of the gods, having a level of control over all the others, although he was not almighty. Some deities had dominion over certain aspects of nature. For instance, Zeus was the sky-god, sending thunder and lightning, Poseidon ruled over the sea and earthquakes, Hades projected his remarkable power throughout the realms of death and the Underworld, and Helios controlled the sun. Other deities ruled over abstract concepts; for instance Aphrodite controlled love. All significant deities were visualized as "human" in form, although often able to transform themselves into animals or natural phenomena.

While being immortal, the gods were certainly not all-good or even all-powerful. They had to obey fate, known to Greek mythology as the Moirai, which overrode any of their divine powers or wills. For instance, in mythology, it was Odysseus' fate to return home to Ithaca after the Trojan War, and the gods could only lengthen his journey and make it harder for him, not stop him.

The gods had human vices and many behaved with arguably less morality than a typical human. They interacted with humans, sometimes even spawning children—called demigods—with them. At times, certain gods would be opposed to others, and they would try to outdo each other. In the Iliad, Aphrodite, Ares, and Apollo support the Trojan side in the Trojan War, while Hera, Athena, and Poseidon support the Greeks (see theomachy).

Some gods were specifically associated with a certain city. Athena was associated with Athens, Apollo with Delphi and Delos, Zeus with Olympia and Aphrodite with Corinth. But other gods were also worshipped in these cities. Other deities were associated with nations outside of Greece; Poseidon was associated with Ethiopia and Troy, and Ares with Thrace.