The crucian carp (Carassius carassius) is a member of the common carp family Cyprinidae.

It is a European species, and its wide range spans from England to Russia; it is found as far north as the Arctic Circle in the Scandinavian countries, and the southern extremities defined by central France and the Black Sea. Its habitat occurs in lakes, ponds, and slow-moving rivers. It has been established that the fish is native to England and not introduced.

The crucian is a medium-sized cyprinid, typically 15 cm in body length, and rarely exceeds in weight over 1.5 kilograms (3.3 pounds),^{[citation needed]}. But a maximum total length of 64.0 cm is reported for a male, and the heaviest published weighed 3 kilograms.

They are broadly described as having a body of "golden-green shining color", but a more precise source states that young fish are golden-bronze but darken with maturity, until they gain a dark green back, deep bronze upper flanks, and gold on the lower flanks and belly, and reddish^{[citation needed]}or orange fins, although other colour variations exist. One distinguishing characteristic is a convexly rounded fin, as opposed to goldfish (or C. gibelio) hybrids which have concave fins.