The Chromis chromis has an oval-shaped, moderately compressed body covered with large scales. It has a small head, relatively large eyes, and a highly protractile mouth adapted for feeding in the water column. This species is diurnal and gregarious, meaning that individuals tend to live and move in groups, often forming large shoals near rocky habitats. Adults are typically grey to black in colour, while juveniles are bright blue when small, measuring up to about 15 mm in length.

During the reproductive season, males prepare nests among rocky substrates and repeatedly dart toward schools of females to attract a mate. When a receptive female responds, the male guides her to the nest, where she lays her eggs and he fertilises them. The female remains with the eggs for a few days before leaving, while the male continues to guard and defend the nest until the eggs hatch.