an organ of chemoreception that is part of the olfactory system (smell) of amphibians (amphibian), reptiles (reptile), and mammals (mammal), although it does not occur in all tetrapod groups. It is a patch of sensory cells within the main nasal chamber that detects heavy moistureborne odour particles. Airborne odours, in contrast, are detected by the olfactory sensory cells located in the main nasal chambers. Some groups of mammals also initiate a behaviour known as the flehmen response, in which the animal facilitates the exposure of the vomeronasal organ to a scent or pheromone by opening the mouth and curling the upper lip during inhalation.

This organ was named for its discoverer, Danish anatomist Ludvig Levin Jacobson, in 1811. It is a paired structure; in the embryo stages of all tetrapods, each half arises as an evagination of the floor of a nasal sac. In fully developed crocodilians (crocodile), turtles (turtle), birds (bird), cetaceans (cetacean), and many advanced primates (primate), this structure is absent or substantially underdeveloped. For most tetrapods that possess a Jacobson's organ, ducts connect the organ directly to the nasal cavity; however, in squamates (lizards (lizard) and snakes (snake)) each organ opens on the roof of the buccal cavity (mouth). The tongue carries odour particles from the outside to the vomeronasal openings on the roof of the mouth, and the particles then move into the vomeronasal organ. After these particles reach the organ, some of the chemical compounds they contain bind to receptor molecules, and sensory messages are sent to the brain.

The Jacobson's organ is useful in the process of communicating chemical messages, such as readiness for sexual activity (sex), between members of the same species. The organ also helps snakes hunt and track their prey. Much evidence suggests that this organ may also be involved in the detection of chemical signals related to aggression and territoriality. See also chemoreception.