The Accessory olfactory system (AOS) is one of the two olfactory systems commonly found in vertebrates. Like the main olfactory system, the accessory olfactory system is a chemosensory system, which transduces chemicals into neural activity. Unlike the main olfactory system, which detects volatile chemicals, the accessory olfactory system detects fluid-phase chemicals. Behavioral evidence suggests that often, the stimuli detected by the accessory olfactory system function as pheromones.

The sensory receptors of the accessory olfactory system are located in the vomeronasal organ. As in the main olfactory system, the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb, located on the dorsal-posterior portion of the main olfactory bulb. Unlike in the main olfactory system, the axons that leave the accessory olfactory bulb do not project to cortex but rather to targets in the amygdala and hypothalamus where they may influence aggressive and mating behavior.