The intrinsic autopodial musculature of mammals is composed of a relatively stable number of muscular groups. Here, we present the first myological studies of Lyncodon patagonicus, a South American mustelid that is morphologically and ecologically poorly known. The typical intrinsic muscles of the manus of Lyncodon are similar to those of its sister taxon Galictis, although there exist few variations, including additional subdivisions and relatively more distal insertions in some muscles. A striking feature is the presence of a novel series of forepaw muscles never before described for mammals. These muscles, named here mm. flexores digitorum proprii manus, are represented by a medial and a lateral belly for each digit and are located immediately distal to the mm. flexores breves profundi. They are located distally alongside the tendons of the latter, along the palmar aspect of the phalangeal series, and they insert onto the flexor tubercle of the distal phalanx of each digit. We propose that the incorporation of these muscles in the forepaw musculature of Lyncodon, in addition to some other osteo-myological features, imply a distinctive morpho-functional adaptation to increase the force of flexion and independence of digital movements likely used to grasp prey during hunting. This hypothesis should be confirmed by future ecological and behavioral studies on this species.