Nodular fasciitis is a benign fibroblastic lesion that was historically misdiagnosed as a malignant neoplasm. Patients present with pain and swelling of relatively brief duration. The clinical presentation is suggestive of an aggressive lesion, usually occurring in muscle fascia. Histologic features can cause it to be mistaken for sarcoma. After the diagnosis is established histologically, observation is the suggested treatment. We present the case of a patient who had a large soft-tissue tumor in the upper arm with a clinical picture indicative of sarcoma, which ultimately was diagnosed as nodular fasciitis. The patient was treated with anti-inflammatory agents and observation. Within 7 months, the mass almost completely resolved, as documented by magnetic resonance imaging.