The trireme, Greek: τριήρης sing., τριήρεις pl., Latin: triremis sing., triremes plural, is a class of warships used by the ancient civilizations of the Mediterranean. They were mainly used by the Phoenicians, ancient Greeks and Romans. In English, no differentiation is made between the Greek triērēs and the Latin triremis. The trireme gets its name from the three rows of oars used to power it through the water. The trireme was an evolutionary step in the development of oar powered warships.

Prior to using three rows of oars were the penteconter, an ancient warship with a single row of 25 oars on each side, and of the bireme, Greek: διήρης, a warship with two banks of oars, probably of Phoenician origin. Then there was the quadrireme surpassed the Trireme in use after the 4th century BC. The Trireme was used almost exclusively from the 7th to 4th century BC.

The superiority of the trireme helped lead to the creation of the Athenian maritime empire prior to and after the Persian War, it also related directly to its downfall during the Peloponnesian War.

Exactly when and how the trireme was developed is not known. There are references to this type of ship dating to the mid 8th century BC. Thucydides states the trireme was introduced to Greece by the Corinthians in the late 8th century BC, and that the Corinthian Ameinocles built four such ships for the Samians. However 2nd century Christian scholar Clement of Alexandria, drawing on earlier works, explicitly attributes the invention of the trireme, trikrotos naus, three-tiered ship, to the Sidonians. There are also references in art work dating to the early 8th century depicting both two and three leveled warship. These come from the Assyrian capital of Nineveh depicting the fleets of Tyre and Sidon.

Construction of the ships was done almost backward from a ship of today. The skin of the ship was built and the ribbing added afterward. Today the ribbing is done fist and the skin is added afterward. The skin of a trireme was put together using the mortise and tenon with pegs method. This made the skin very tight and added strength. The ships were roughly 37 meters long and 4 meters tall. The draft of a trireme was roughly 1 meter. This allowed the ships to be easily beached, but rough handling in heavy seas.

Classical accounts of the triremes speed place it around 6 knots up to 9 knots. On trials conducted on the modern reconstructed version speeds range from 4 knots to 7 knots. 170 men were used to man to oars on a large trireme. Two small sails were also used to provide forward speed. The main weapon of a Trireme was its ram. The ram protruded at the waterline and was normally encased in metal, mostly copper.

Tactics for attacking other naval vessels at the time included ramming enemy ships and puncturing the hull. Also raking the sides of the ships and breaking as many oars as possible. This of course would immobilize a ship and make it an easy target. Some ships would be fitted with catapults or ballistas, this was mainly in the later centuries. Hand to hand fighting would also be used to take an enemy ship once it was immobilized. And one final weapon used by navies at the time, fire.

Over time just as the Trireme had done to the bireme the quadreme gradually replaced her as the dominate ship. Naval designers were always on the lookout for fast stronger ships. The basic design of the trireme was carried through for many centuries to come. The Viking long boat is said to have roots dating back to the trireme, although it used only one set of oars. Oar power remained as a source of power well into the middle ages where sail power eventually over took oars.