The Battle of Pylos took place between Athens and Sparta during the Peloponnesian War in 425 BC. Accompanying this battle was the Battle of Sphacteria.

In the spring of 425 BC, Sparta helped Messina in Sicily revolt from Athens, and also launched another invasion of Attica under the command of King Agis. The Athenians sent forty ships to Sicily under the command of Eurymedon, Sophocles, and Pythodorus, but allowed them to divert to Corcyra to meet up with Demosthenes, who had recently won the Battle of Olpae. Demosthenes had them land at Pylos rather than complete the journey to Corcyra, as he had a plan to fortify Pylos and thereby gain a foothold in the Peloponnese close to Sparta. The other commanders thought this was a waste of time and money, but the soldiers found that fortifying the land was a useful diversion from the bad weather that was keeping the ships from sailing. Within six days Pylos was fortified, and Demosthenes remained there with five ships while the main fleet continued on their missions to Corcyra and Sicily.

When Sparta learned that Athens had taken Pylos they removed their army from Attica. They marched to Pylos and had a fleet of sixty ships meet them there; Demosthenes had the forty Athenian ships return when he learned of this fleet. The Spartans planned to blockade the port of Pylos and land an army on the nearby island of Sphacteria, so that the Athenians would have no base to supply their troops. The Spartan commander Epitadas and a force of 440 hoplites were landed on Sphacteria. Demosthenes had fewer hoplites, and most of the rest of his troops were unarmed sailors from the remaining triremes. He took sixty of his hoplites to the unfortified beach so there would be some defense against a Spartan landing.

In a famous speech to his troops, Demosthenes noted an oft-proved ancient principle of amphibious warfare: if you stand your ground on the beach, the enemy cannot break through your lines. The only thing the Athenian forces had to fear (to quote a certain famous twentieth-century politician) was fear itself. Forty-three Spartan ships, under Thrasymelidas and Brasidas, attempted to force a landing, but Brasidas was injured, and the Spartan ships were pushed back by the Athenian land force.

After three days the rest of the Athenian fleet arrived, now with fifty ships that had joined them on the way. Although the Spartans would not meet them for a battle at sea, they had also not properly blockaded the entrance to Pylos, and the Athenian fleet was able to enter and chase away and destroy the attacking Spartan ships. This meant that the hoplites on Sphacteria were now cut off from the mainland.

Spartan ambassadors were sent to Athens to negotiate a truce, and the Athenians remained at Pylos for a total of seventy-two days, during which time the ambassadors failed to make peace and the Athenians finally invaded Sphacteria. See Battle of Sphacteria for this portion of the siege.