Astacus (from the Greek αστακός, astacós, meaning "lobster" or "crayfish") is a genus of crayfish found in Europe, comprising three extant (living) species and three somewhat contested extinct species. These crayfish are found across Europe and live exclusively in freshwater, mostly inhabiting the bottom of lakes, ponds, and streams.

Due to the crayfish plague, crayfish of this genus have declined in many European regions, being replaced by the invasive North American signal crayfish, which carries the plague but is unaffected by it.

Astacus was the first named genus of freshwater crayfish, with many former species later being reassigned to other genera.

Astacus belongs to the family Astacidae, one of the three families of Northern Hemisphere freshwater crayfish within the superfamily Astacoidea. The internal phylogeny of Astacidae can be shown in the cladogram below:

"A. astacus have a dark brown or black coloration on their dorsal side, and an olive-brown or reddish-brown coloration on their ventral side. Their carapace is smooth, with small granular tubercles along the edges. The rostrum is flat and has two spines near its base. The claws are broad with well-defined bases, reddish-brown on the ventral side, and rough on the dorsal side."

They have been found to span the entire continent and prefer temperate water ranging from 12°C to 20°C, but they are also able to survive during colder periods when temperatures are closer to freezing.

The genus has been known to eat almost anything, typically crawling along the bottom of a body of water searching for another meal. They can be defined as detritovores since they persist mostly on decaying plant matter as well as dead or dying animals. This unique diet makes these crayfish essential for maintaining clean and healthy waterways as they clear out decaying matter and consume fish that are affected by illness or disease.

As is the case with all arthropods/ecdysozoans, the genus Astacus must molt their cuticle layer in order to grow and develop.