Intramedullary osteosynthesis implant, permitting in particular arthrodesis (1) of a joint, for example an interphalangeal joint, or diaphyseal osteosynthesis of the upper limb or of the lower limb, comprising two sets of at least two rods (2, 3, 4, 5) each extending on either side of a central zone (6), said rods (2, 3; 4, 5) being substantially parallel at ambient temperature within the same set, each set of rods being intended to be impacted in the medullary canal of a diaphysis, said implant being made from a shape-memory material so that, at body temperature, the rods (2, 3; 4, 5) of the same set spread apart so as to be able to immobilize themselves in said medullary canal.