In ancient Greek religion and mythology, Achelous (also Acheloos or Acheloios) (ˌækᵻˈloʊ.əs; 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.
The name Ἀχελώϊος is possibly pre-Greek, its meaning is not entirely certain. Recent arguments suggest it is Semitic in origin, with the initial Αχ- stemming from the Akkadian aḫu ("bank of the river"), or aḫû ("seashore") and the suffix -ελώἴος, from the Akkadian illu ("watercourse" or "water of the river invading land"). Exact match of the root achel- can be found in the Thracian language, with meaning "water".
According to Hesiod, Achelous, along with all the other river gods, was the son of the Titans Oceanus and Tethys. According to the sixth-century mythographer Acusilaus, Achelous was the "oldest and most honoured" of the river-god offspring of Oceanus. Servius relating a tradition of unknown origin, reports that Achelous was said to have been the son of Earth (i.e. Gaia). The Renaissance mythographer Natalis Comes wrote that Alcaeus understood Achelous to be the son of Ocean and Earth.
Achelous had various offspring. He was said to be the father of the Sirens. According to the 3rd-century BC poet Lycophron, the Sirens were the daughters of Achelous, by an unnamed "melodious mother" (perhaps meaning the mother was a Muse). Another 3rd-century BC, poet Apollonius of Rhodes, makes the mother the Muse Terpsichore, while according to other accounts, she was the Muse Melpomene, or the Calydonian princess Sterope. Ovid calls the Sirens simply daughters of Achelous, with no mention of their mother. By Perimede, the daughter of Aeolus, Achelous was said to have fathered Hippodamas and Orestes.