The Diolkos (Δίολκος, from the Greek dia διά, "across", and holkos ὁλκός, "portage machine") was a paved trackway near Corinth in Ancient Greece which enabled boats to be moved overland across the Isthmus of Corinth. The shortcut allowed ancient vessels to avoid the long and dangerous circumnavigation of the Peloponnese peninsula. The phrase "as fast as a Corinthian", penned by the comic playwright Aristophanes, indicates that the trackway was common knowledge and had acquired a reputation for swiftness.

The main function of the Diolkos was the transfer of goods, although in times of war it also became a preferred means of speeding up naval campaigns. The 6-to-8.5-kilometre-long (3+3⁄4 to 5+1⁄4 mi) roadway was a rudimentary form of railway, and operated from c. 600 BC until the middle of the first century AD. The Diolkos combined the two principles of the railway and the overland transport of ships, on a scale that remained unique in antiquity.

The Diolkos saved ships sailing from the Ionian Sea to the Aegean Sea, a dangerous sea journey round the Peloponnese, whose three headlands had a reputation for gales, especially Cape Matapan and Cape Malea. By contrast, both the Gulf of Corinth and the Saronic Gulf were relatively sheltered waters. In addition, the overland passage of the Isthmus, a neck of land 6.4 km (4.0 mi) wide at its narrowest, offered a much shorter route to Athens for ships sailing to and from the Ionian coast of Greece.

The Diolkos also had a commercial function in transporting goods. Little is known of its success in increasing trade, but, because of the length of time it was maintained, it is presumed that it had some positive impact. In addition to trade, during wartime the Diolkos may have been used to transport lighter ships across land.

Ancient literature is silent on the date of the construction of the Diolkos. For Thucydides (460–395 BC) the Diolkos already seemed to be something ancient. Excavated letters and associated pottery found at the site indicate a construction date at the end of the 7th or beginning of the 6th century BC, that is around the time when Periander was tyrant of Corinth.

The Diolkos remained reportedly in regular service until at least the middle of the 1st century AD, after which no more written references appear. Possibly the trackway was put out of use by Nero's abortive canal works in 67 AD. Much later transports of warships across the Isthmus in the late 9th century, and around 1150, are assumed to have used a route other than the Diolkos, due to the extensive time lag.

The Diolkos played an important role in Ancient Greek naval warfare. Greek historians note several occasions from the 5th to the 1st century BC when warships were hauled and pulled across the Isthmus in order to speed up naval campaigning. In 428 BC, the Spartans planned to transport their warships over the Diolkos to the Saronic Gulf to threaten Athens, while later in the Peloponnesian War, in 411 BC, they carted over a squadron heading quickly for operations at Chios. In 220 BC, Demetrius of Pharos had a fleet of about fifty vessels dragged across the Isthmus to the Bay of Corinth by his men.

Three years later, a Macedonian fleet of 38 vessels was sent across by Philip V, while the larger warships sailed around Cape Malea. After his victory at Actium in 31 BC, Octavian advanced as fast as possible against Marc Antony by ordering part of his 260 Liburnians to be carried over the Isthmus. In 868 AD, the Byzantine admiral Niketas Oryphas had his whole fleet of one hundred dromons dragged across the Isthmus in a quickly executed operation, but this took place most likely on a different route.