The Axios is the longest and major river in the Republic of Macedonia and also a major river of Greece. It is 388 km (241 mi) long, and drains an area of around 25,000 km2 (9,653 sq mi). The maximum depth of the river is 4 m (13 ft).

The river rises at Vrutok, a few kilometers north of Gostivar in the Republic of Macedonia. It passes through Gostivar, Skopje and into Veles, crosses the Greek border near Gevgelija, Polykastro and Axioupoli ("town on the Axiós"), before emptying into the Aegean Sea in Central Macedonia west of Thessaloniki in northern Greece.

The Vardar basin includes two-thirds of the territory of the Republic of Macedonia. The area is sometimes called "Vardar Macedonia" after the river, to distinguish it from Greek Macedonia and Blagoevgrad Province, also known as "Pirin Macedonia," in Bulgaria.

The valley comprises fertile lands in Polog, and Thessaloniki prefectures and Gevgelija and other parts. The river is surrounded by mountains elsewhere. The superhighways Greek National Road 1 in Greece and M1 and E75 run within the valley along the river's entire length to near Skopje.

The river has been very famous in the Ottoman Empire and modern-day Turkey and was inspiration for many folk songs, most famous of which is the song Vardar Ovasi.