A young, intact, male Bernese Mountain Dog was presented to the animal hospital for lameness and diffuse thickening of the soft tissue in the right hind limb. Magnetic resonance imaging revealed multiple, multilobular, space-occupying lesions within and between the muscles of the right femur. Biopsies taken from the lesions revealed an infiltrative mass composed mainly of collagen fibers and a low density of benign-appearing fibroblasts. These findings were compatible with a diagnosis of a fibromatosis. Taking the age of onset into account, infantile fibromatosis was most likely. A deep fibromatosis, similar to that seen in adults, could not be excluded based on histology.