Introduction: Sinonasal undifferentiated carcinoma (SNUC) is a rare and aggressive malignancy of the nasal and paranasal sinus epithelium, characterized by rapid progression and poor prognosis. Diagnosing SNUC is challenging and often a diagnosis of exclusion when histopathological features do not meet criteria for other sinonasal carcinomas. Advances in understanding newly described neoplasms (e.g. NUT midline carcinoma) and subsets of SNUC (e.g. SMARCB1-deficient sinonasal carcinoma) suggest that cases historically labelled as SNUC may represent distinct entities under current pathology criteria. Aim: This study re-evaluates historical tissue samples from patients diagnosed with SNUC in Sydney, Australia, over 24 years. Using contemporary diagnostic criteria and techniques, we aim to classify these cases into newly recognized subsets or alternative diagnoses, improving understanding of treatment options, responses, and prognoses. Methods: Archival tumour specimens (n = 23) with a historical diagnosis of SNUC were subjected to histopathological re-review, immunohistochemical analysis, and molecular profiling via next-generation sequencing. Findings were correlated with clinical presentations, treatment courses, and outcomes. Results: Re-evaluation using advanced diagnostic tools demonstrated the potential to refine the classification of sinonasal malignancies previously diagnosed as SNUC. Modern histopathological and molecular techniques highlight the evolving diagnostic landscape and the importance of aligning historical diagnoses with current knowledge. Conclusion: This study highlights the value of retrospective analyses in rare malignancies such as SNUC. Revisiting archived specimens with advanced tools refines diagnostic accuracy, enhances patient stratification, and informs therapeutic strategies. Collaborative research is essential to validate these findings and improve clinical practice for rare sinonasal cancers.