Phormio, the son of Asopius, was an Athenian general and admiral during the Peloponnesian War. Nothing is known of him except for the information provided by Thucydides in his History of the Peloponnesian War.

At the beginning of the Peloponnesian War he was assigned to lead the Athenian troops against Potidaea. He blockaded the town and pillaged the surrounding land. Later in 432 BC he shared command of the Athenian fleet with Thucydides and Hagnon against Samos.

In 431 BC he led 1600 Athenians against Chalcidice, along with Perdiccas of Macedon. In 430 BC he led the Athenian fleet sent to help Ambracia and Acarnania at the Battle of Potidaea. He stationed the fleet of twenty ships at Naupactus to blockade Corinth. Phormio defeated the Corinthian fleet at the Battle of Naupactus in 429 BC, trapping them in the narrow strait and capturing twelve of their ships. A larger Corinthian and Spartan fleet attempted to attack him again a few days later, and although he lost many of his own ships he forced them to withdraw for a second time.

After these victories he landed 400 Athenians in Acarnania, engaged in some fighting with the inhabitants, and then returned to Athens in 428 BC.

His son, also named Asopius, also commanded naval expeditions during the war. Phormio is not mentioned after 428.

Another Phormio was a freed slave who was the subject of a speech by Demosthenes.

Phormio is also a comedy by the Roman playwright Terence.