In Greek mythology, Achelous was the patron deity of the river by the same name, which is the largest river of Greece, and thus the chief of all river deities, every river having its own river spirit. His name translates as "he who washes away care". He was the eldest child of Oceanus and Tethys. Achelous was a suitor for Deianeira, daughter of Oeneus king of Calydon, but was defeated by Heracles, who wed her himself. Sophocles pictures a mortal woman's terror at being courted by a chthonic river god:
'My suitor was the river Achelóüs,
who took three forms to ask me of my father:
a rambling bull once, then a writhing snake
of gleaming colors, then again a man
with ox-like face: and from his beard's dark shadows
stream upon stream of water tumbled down.
Such was my suitor.' (Sophocles, Trachiniae)

The sacred bull the snake and the Minotaur are all creatures associated with the Earth Goddess Gaia. Achelous was also portrayed as a gray-haired old man with horns. He was also considered a storm-god. He was sometimes the father of the Sirens by Terpsichore.

When Achelous was defeated, Heracles took one of his horns, and Achelous had to trade the goat horn of Amalthea to get it back. Heracles gave it to the Naiads, who transformed it into the cornucopia.

The Achelous river formed the boundary between Acharnania and Aetolia of antiquity. It empties into the Ionian Sea. In another mythic context, the Achelous was said to be formed by the tears of Niobe, who fled to Mt. Sipylon after the deaths of her husband and children.

The mouth of the Achelous river was the spot where Alcmaeon finally found peace from the Erinyes. Achelous offered him Callirhoe, his daughter, in marriage if Alcmaeon would retrieve the clothing and jewelry his mother, Eriphyle, had been wearing when she sent her husband, Amphiaraus to his death. Alcmaeon had to retrieve the clothes from King Phegeus, who sent his sons to kill Alcmaeon.

Ovid, Metamorphoses, VIII, 547, IX, 1, and X, 87.