Ultrasonography of the temporomandibular joint is a non-invasive imaging technic, easy to perform in daily practice. It can be used for diagnosis and to guide intra-articular injections. The objective was to validate a longitudinal in-plane US injection approach of the joint and assess its accuracy. We performed a study in 13 non-embalmed cadavers. The injection was done under real-time US guidance using a needle inserted in-plane with an angulation of 30°and positioned under the capsule until the injection was feasible without resistance. The intra-articular injection was successful in all cases and confirmed by a liquid backflow in 96% of cases. The median duration between skin puncture and the intra-articular injection was 23 seconds. Our technique allows a direct visualization of the needle throughout its course to the joint with a high accuracy. Other studies will be needed to confirm its feasibility and usefulness in patients with TMJ disorders.