The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes collectively referred to as the Weberian ossicles or Weber's ossicles. The presence of the structure is one of the most important and phylogenetically significant distinguishing characteristics of the Ostariophysi. The structure itself consists of a set of minute bones that originate from the first few vertebrae to develop in an embryonic ostariophysan. These bones grow to physically connect the auditory system, specifically the inner ear, to the swim bladder. The structure acts as an amplifier of sound waves that would otherwise be only slightly perceivable by the inner ear structure alone.

The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish to control their buoyancy, and thus to stay at their current water depth without having to expend energy in swimming. Also, the dorsal position of the swim bladder means that the expansion of the bladder moves the center of mass downwards, allowing it to act as a stabilizing agent in some species. Additionally, the swim bladder functions as a resonating chamber, to produce or receive sound.