Alcathous was the name of several people in Greek mythology:
- Alcathous, son of Pelops, killed the Cithaeronian lion.
- Alcathous, son of Aesyetes, husband of Hippodameia, the daughter of Anchises and sister of Aeneas, who was educated in the house of Alcathous. In the Trojan War, he was one of the Trojan leaders, and was one of the handsomest and bravest among them. He was slain by the Cretan king Idomeneus with the assistance of Poseidon, who struck Alcathous with blindness and paralyzed his limbs so that he could not flee.
- Alcathous, son of Porthaon and Euryte, daughter of Hippodamas. He was killed by Tydeus, the son of Oeneus, Alcathous' brother. In another story, he was instead killed by Oenomaus, being one of the unsuccessful suitors of Hippodamia.
- Alcathous, one of the companions of Aeneas. He was killed by Caedicus, one of the warriors of Turnus.
- Alcathous, one the guardians of Thebes. He was killed by Amphiaraus.
- Alcathous, a Trojan warrior, killed by Achilles in the Trojan War.
- Alcathous, another, otherwise unknown personage of this name is mentioned by Virgil.
Alcathous fecialis is a species of fulgorid planthopper found in south India. It is the sole species in its genus. It is predominantly brown with dark markings. The snout curves slightly upward. The wings are about 3 cm long and hide the short abdomen. From the eyes to the tip of the abdomen, they are about 1.2 cm long with a 0.3 cm snout.