Recent advances in novel immunotherapeutic and targeted therapeutic agents have increased treatment options in patients with advanced metastatic melanoma. However, evidence in the literature on whether or not extracutaneous melanoma will acquire an equivalent advantage from these therapies is very scarce. In general, extracutaneous melanomas are rare and aggressive melanomas, which are clinically and biologically unique, with higher incidence of metastatic disease and poor prognosis. In this case report, we present a very rare case of a 54-year-old woman with primary pelvic retroperitoneal melanoma treated with an anti-PD-1 antibody, pembrolizumab. Furthermore, we explore the role of novel immunotherapies in the treatment of advanced melanoma.