The invention describes an immobilisation device (1) for radiotherapy comprising: an inclined plate (2) for supporting the back of a patient, wherein said plate (2) is inclined in a longitudinal direction and has a lower end (21) and an upper end (2u)； and a lower platform (3) for supporting the buttocks of the patient, wherein said lower platform (3) can be coupled to the lower end (21) of the inclined plate (2), wherein the lower platform (3) is rotatable to make it easier to place the patient on the device (1). The lower platform can also comprise a protuberance (4) for supporting the perineum of the patient and plates (5) for immobilising their hips.
