The naval Battle of Eretria, between Sparta and Athens, took place in September 411 BC, off the coast of Euboea.
During the spring of 411 BC, the Eretrians drove the Athenians out of Oropos with the help of the Boeotians.
By the end of the summer 411 BC, a large Spartan fleet sailed towards Euboea.
The Athenians tried to prevent the Euboeans from switching sides by sending a squadron to Eretria under Thymochares.
While the Athenians were in the harbour of Eretria in order to supply themselves, the Eretrians informed the Spartan admiral Hegesandridas by a signal fire that it was an appropriate time to attack.