In ancient Greek religion and mythology, Achelous (also Acheloos or Acheloios) (; Ancient Greek: Ἀχελώϊος, and later Ἀχελῷος, Akhelôios) was the god associated with the Achelous River, the largest river in Greece. According to Hesiod, he was the son of the Titans Oceanus and Tethys. He was also said to be the father of the Sirens, several nymphs, and other offspring.

Achelous was able to change his shape, and in the form of a bull, he wrestled Heracles for the right to marry Deianeira, but lost. He was also involved in the legend of the Argive hero Alcmaeon.