The invention concerns an anterior osteosynthesis plate for lumbar vertebrae or sacral lumbar vertebra comprising: an elongated body with the following features: adapted to cover the anterior part of two consecutive vertebrae; having an anterior surface (6) and an opposite posterior surface (7); presenting two end zones linked by a connecting zone (11); and provided with two passage holes for anchoring screws. The invention is characterised in that the plate posterior surface (7) has a support plate (12, 13) designed to be urged in contact with the anterior surface of the vertebrae, the support plates (12, 13) being inclined to define between them, along a saggital plane (S), an obtuse angle (α) adapted to the relative angulation of the anterior part of the two vertebrae and mutually linked at the connecting zone by a concave extension (17), such that at least the posterior surface (7) of the body presents a concave profile in the saggital plane.