Dienekes (Διηνέκης) was a Spartan officer present at the Battle of Thermopylae. He was acclaimed the bravest of all the three hundred Spartiates selected to fight in that battle.

Herodotus related the following anecdote about Dienekes:

Thus nobly did the whole body of Lacedaemonians and Thespians behave; but nevertheless one man is said to have distinguished himself above all the rest, to wit, Dieneces the Spartan. A speech which he made before the Greeks engaged the Medes, remains on record. One of the Trachinians told him, “Such was the number of the barbarians, that when they shot forth their arrows the sun would be darkened by their multitude.” Dieneces, not at all frightened at these words, but making light of the Median numbers, answered “Our Trachinian friend brings us excellent tidings. If the Medes darken the sun, we shall have our fight in the shade.” Other sayings too of a like nature are reported to have been left on record by this same person.
Histories, 7.226

Dienekes is one of the main characters in Steven Pressfield's novel Gates of Fire.