Synovial joints are the most common and flexible type of joint in mammals. These freely moveable joints are characterized by the presence of a synovial cavity, articular cartilage and joint capsule. A widely held view is that synovial joints first evolved in tetrapods, with fish lacking this type of joints. Surprisingly however, we found that joints in the jaw and pectoral fin of zebrafish, stickleback and spotted gar share the characteristics of mammalian synovial joints. Histological analysis and live imaging of fluorescent transgenic zebrafish from 3 days post fertilization to 12 months of age demonstrated that the zebrafish jaw joint has a synovial cavity and is wrapped in a thin joint capsule. Cartilage cells that have flattened morphology just like mammalian articular chondrocytes cover the surface of juxtaposed skeletal elements at the zebrafish jaw and fin joints. In situ hybridization analysis of genes expressed in mammalian synovial joints revealed that, similar to mammals, the zebrafish synovial joint articular cells do not express collagen type II or X, aggrecan or matrilin1, and instead express a joint lubricating proteoglycan, lubricin, encoded by the gene prg4b. Lubricin is secreted from the articular cells of the joint to lubricate the joint surface and its expression is one of the hallmarks of a synovial joint. To determine whether the jaw joint prg4b expression pattern is a conserved feature throughout ray-finned fish species, we investigated prg4 expression in a distantly related teleost fish, the threespine stickleback (Gasterosteus aculeatus), and a basal ray-finned fish, the spotted gar (Lepisosteus oculatus). Similar to zebrafish, prg4 homolog expression is restricted to the jaw joint articular surface in juvenile stickleback and spotted gar. Finally, genetic deletion of zebrafish prg4b demonstrated a conserved requirement for prg4b gene function in the age-related maintenance of joint cartilage. Our data represent the first molecular and functional evidence that ray-finned fish have lubricated synovial joints, supporting a model that lubricated synovial joints evolved at least in the last common ancestor of all bony vertebrates. Moreover, our results establish the zebrafish as a novel model organism to investigate synovial joint development and repair, and opens up the powerful genetic tools of the zebrafish to the field of arthritis research.