The Haliacmon (Greek: Αλιάκμονας, Aliákmonas; formerly: Ἁλιάκμων, Aliákmon or Haliákmōn) is the longest river flowing entirely in Greece, with a total length of 297 km (185 mi).

This tradition is confirmed by the following record of the Roman author Pliny the Elder (23-79 AD) : Similarly in Macedonia, those who want their sheep to be white go to Haliakmon, while those (who want them to be) black (go) to Axios (Vardar).

It feeds the large artificial lake Polyfyto, that was created after the construction of the namesake hydroelectric dam and consists almost its entire course through the Kozani prefecture.

Southeast of Veria, the Haliacmon enters the central Macedonian plains, an area of great importance to agriculture.

The Haliacmon flows along the towns (in downstream order) Nestorio, Argos Orestiko, Neapoli, Paliouria, Velventos and Alexandreia.

These include brown trout, gilt-head bream, sardine, carp, pike, eel, european anchovy, nursehound, bogue, garfish, red porgy, saddled seabream, angler, bream, mediterranean sand smelt, sand steenbras, mullus barbatus, freshwater bass, tuna, salmon, Mediterranean moray, weever, longfin gurnard, dogfish, dusky grouper, school shark and turbot.

In the place where the river flows into the sea, there has been formed over the years an extended Delta of 4.000 hectares, because of the large dam that was constructed and retains much of the brought matters.

As a result, silting has been greatly reduced and during the summer, when there is not much water, the sea enters and largely overwhelms the river bed.

In the shallow marine areas formed there, the fry of many Aegean Sea's fish finds a place to reproduce, while about 90% of Greece's mussels are produced in the Haliakmon-Axios (Vardar) Delta.

In the upper part of its course it took a southeast direction through Elimiotis, which it watered; and then, continuing to the northeast, formed the boundary between Pieria, Eordaea, and Imathia.