The Battle of Delium took place in 424 BC between the Athenians and the Boeotians, and ended with the siege of Delium in the following weeks.

In 424 the Athenian generals Demosthenes and Hippocrates planned to invade Boeotia. Demosthenes mistakenly sailed too early and landed at Siphae , where his plans were betrayed by a Phocian named Nicomachus. As Hippocrates had not yet arrived, Demosthenes could not attack and was forced to withdraw.

Hippocrates eventually did arrive in Boeotia with an Athenian army and began to fortify the temple at Delium. After five days, the fortifications were complete, and Hippocrates set up a garrison and sent the rest of his army back to Athens. At the same time, the Boeotians gathered their army to challenge Hippocrates, but when they saw that the Athenians were leaving many of them thought it was pointless to attack. Pagondas of Thebes, the commander of the Boeotian forces, urged them to attack anyway, because he knew the Athenians would eventually return and use Delium as a base for further invasions. Pagondas moved his army into position near the Athenians, although both armies were hidden from each other by a hill. The Boeotians had 7000 hoplites, 1000 cavalry, 500 peltasts, and 10 000 light troops, forming a line 25 men deep. The right wing was formed by troops from Thebes, the centre by men from Haliartia , Coronaea , and Copaea , and the left wing by troops from Thespiae, Tanagra, and Orchomenia . They were later joined by the Locrians . When Hippocrates learned of the Boeotian army, he joined the main Athenian force, leaving 300 cavalry behind at Delium. The Athenians had about the same numbers of hoplites and cavalry, but had fewer lightly armed troops, mostly from their allied cities. One of the Athenian soldiers in the battle was the philosopher Socrates.

The Boeotians charged unexpectedly while Hippocrates was giving a speech to his men. The centre lines saw the heaviest fighting. The Boeotian left wing was surrounded and defeated, but Pagondas sent some cavalry to support them and the Athenians were defeated. Meanwhile, the Boeotian right wing was also victorious, and the Athenians fighting there fled; when the Athenian centre saw that their two wings had been defeated they also fled. About 500 Boeotians and 1000 Athenians had been killed, including Hippocrates.

The Boeotians chased the Athenians until nightfall. Most of the Athenians returned to the fort at Delium, where a Boeotian herald announced that they were offending land sacred to the Boeotians and must leave. The Athenians replied that the land was now theirs and was now sacred to them, and that they held it in self-defense from the Boeotians, who were allies of the Spartans.

For two weeks there was no action, but the Boeotians were joined by 2000 hoplites from Corinth, as well as other troops from their various allies. The Boeotians constructed a strange device, which according to the description in Thucydides (4.100) seems to be a kind of flamethrower, and used this weapon to set fire to Delium and chase away the Athenians. Only about 200 Athenians were killed; the rest were allowed to escape.

After Delium had been recaptured, Demosthenes and his forces finally arrived, but the lack of communication between him and Hippocrates meant that his arrival was essentially useless. He landed near Sicyon, but was quickly defeated.