The preopercle is a bony gill cover present in fishes. It articulates with the opercle, and has a free margin that bears spines or serrations. The presence of a preopercle is one of the main features used to place fishes into families. For example, the serrated preopercle of wrasses distinguishes them from similarly coloured labrids. In many perciforms, the preopercular spine is enlarged and projects forward over the lower jaw as a kind of latch. This helps to keep the mouth shut when feeding on zooplankton or small bottom-dwelling organisms.
The term preopercle can also refer to a soft tissue flap located behind the operculum in some fishes (e.g., sculpins). This flap may be used to funnel water towards the gills, or it may serve some other function.