Hematopoietic neoplasia is common in dogs, with canine non‐Hodgkin lymphomas representing more than 80% of all hematopoietic cancer. However, extranodal infiltration of the skeletal muscle by non-Hodgkin lymphoma is rare in humans and dogs. A 9 yr old neutered male English mastiff presented with a 3 wk history of recurrent stranguria, pelvic limb ataxia, and mild proprioceptive deficits bilaterally, worse in the right pelvic limb. MRI showed an expansile ill-defined lesion within the bulbospongiosus muscle. The lesion had intermediate signal intensity to muscle and fat on T2-weighted imaging and was isointense to unaffected muscle on precontrast T1-weighted imaging. Contrast enhancement was heterogeneous and there was digitate signal alteration within adjacent perilesional fat. Ultrasound examination confirmed a hypoechoic lesion infiltrating the muscle. Cytological examination yielded a diagnosis of high-grade lymphoma. This report provides the first description of MRI findings associated with cytologically confirmed lymphoma of the skeletal muscle in the dog. Although nonspecific, the imaging features strongly correlate with those in the medical literature and lymphoma should be considered a pertinent differential in cases presenting with similar imaging findings.