The zebrafish (Danio rerio) provides a powerful model for analyzing genetic contributors to cancer. Multiple zebrafish lines with cancer-associated genetic mutations develop soft tissue sarcomas that are histologically consistent with malignant peripheral nerve sheath tumor (MPNST). The goal of this study was to determine the phenotype of soft tissue sarcomas in a brca2-mutant/tp53-mutant zebrafish line using immunohistochemical markers that are commonly expressed in mammalian MPNST. We classified 70 soft tissue sarcomas from a brca2-mutant/tp53-mutant zebrafish cohort as MPNST, undifferentiated sarcoma, or other tumor based on histologic features. The expression of S100, CD57, and glial fibrillary acidic protein (GFAP) was analyzed in nonneoplastic neural tissues and tumor specimens by immunohistochemistry. Each marker was expressed in nonneoplastic neural tissues. In MPNST, S100 and CD57 were widely expressed in neoplastic cells, with greater consistency observed for CD57 expression. In undifferentiated sarcomas, results were variable and correlated to anatomic location. Coelomic undifferentiated sarcomas often exhibited widespread CD57 expression but limited S100 expression. In comparison, ocular undifferentiated sarcomas exhibited limited expression of both CD57 and S100. Overall, CD57 and S100 expression was significantly higher in MPNST than in undifferentiated sarcomas. GFAP was not expressed in any of the tumors. This study identified commercially available antibodies that are useful for analyzing S100, CD57, and GFAP expression in zebrafish. This study further shows a correlation between degree of histologic differentiation and expression of these markers in soft tissue sarcomas from brca2-mutant/tp53-mutant zebrafish and suggests that these cancers are derived from the neural crest with differentiation toward myelinating Schwann cells.