Intramuscular lipomas are rare benign soft tissue tumors that originate within skeletal muscle fibers and account for a small fraction of all lipoma cases. Their infiltrative growth pattern can pose diagnostic challenges, particularly in differentiating them from well-differentiated liposarcomas. We report a case of a 42-year-old male who presented with a painless, gradually enlarging mass over the lateral aspect of the left shoulder. Magnetic resonance imaging (MRI) revealed a well-defined intramuscular fatty lesion within the deltoid muscle, measuring 30 × 19 mm, consistent with a lipomatous tumor. The patient underwent complete surgical excision of the mass. Histopathological examination confirmed an infiltrating intramuscular lipoma composed of mature adipocytes without cellular atypia or malignancy. The postoperative recovery was uneventful, and no recurrence was observed during follow-up. This case highlights the importance of imaging and histological evaluation in distinguishing benign infiltrating lipomas from low-grade liposarcomas. Complete surgical excision remains the treatment of choice to prevent recurrence and ensure definitive diagnosis.