Biphenotypic sinonasal sarcoma (BSNS) is a distinctive, anatomically restricted, low-grade spindle cell sarcoma that shows considerable histologic overlap with other cellular spindle cell neoplasms. This tumor type shows both myogenic and neural differentiation, which can be demonstrated by immunohistochemistry; however, the available diagnostic markers are relatively nonspecific. BSNS is characterized by PAX3 rearrangements, with MAML3 as the most common fusion partner. Our aim was to determine whether immunohistochemistry using a monoclonal PAX3 antibody could distinguish BSNS from potential histologic mimics, as well as to evaluate a widely available polyclonal PAX8 antibody, which is known to cross-react with other paired box transcription factor family members. Immunohistochemistry for PAX3 and PAX8 was performed on whole sections of 15 BSNS (10 with confirmed PAX3 rearrangement) and 10 cases each of the following histologic mimics: malignant peripheral nerve sheath tumor, monophasic synovial sarcoma, spindle cell rhabdomyosarcoma (RMS), solitary fibrous tumor, sinonasal hemangiopericytoma, and cellular schwannoma, as well as alveolar RMS (which harbors PAX3 or PAX7 gene rearrangements). BSNS showed consistent expression of PAX3 (15/15), all multifocal-to-diffuse and most with moderate-to-strong intensity of staining. One single case of spindle cell RMS showed PAX3 expression (1/10), and all other histologic mimics were completely PAX3-negative. In contrast, nuclear staining for PAX8 was present in all 15 BSNS, 7/10 malignant peripheral nerve sheath tumor, 3/10 cellular schwannomas, 2/10 sinonasal hemangiopericytomas, 1/10 synovial sarcoma, 1 spindle cell RMS, and 1 solitary fibrous tumor. All cases of alveolar RMS were positive for PAX8, and most were also positive for PAX3 (8/10). Immunohistochemical expression of PAX3 is highly sensitive (100%) and specific (98%) for BSNS. A polyclonal PAX8 antibody also stains BSNS (likely due to cross-reactivity with PAX3) but has much lower specificity (75%), with frequent expression in numerous mimics.