Atypical fibroxanthoma (AFX) is a rare cutaneous fibrohistiocytic tumor that typically arises on chronically sun-damaged skin, such as the head and neck, as a nondescript ulcerated papule, nodule, or tumor. The clinical prognosis is usually favorable and metastasis is rare. Pleomorphic dermal sarcoma (PDS), or undifferentiated pleomorphic sarcoma, is a recently introduced diagnostic moniker for AFX-like tumors with more aggressive clinical and histologic features such as necrosis and vascular invasion. The exact relationship between AFX and PDS has been debated. Diagnosis of these tumors is generally based on immunohistochemical staining to exclude other mimics. A wholly specific marker for this tumor does not exist, leading to diagnostic ambiguity in certain cases. Herein, we present a case of pleomorphic dermal sarcoma in a 53-year-old man with human immunodeficiency virus that displayed patchy S100 staining concerning for melanoma upon hospital pathology review. Next-generation sequencing analysis confirmed a mutation pattern consistent with published molecular signatures of AFX/PDS. In discussing this case, we review the current understanding of AFX/PDS and discuss diagnostic pitfalls, as well as emphasize on how next-generation sequencing techniques might improve accuracy in the diagnosis of tumors in the spectrum of AFX/PDS.