Aims: Extramammary Paget disease (EMPD) is an epithelial neoplasm that can occur at many sites, including the vulva and scrotum. EMPD is characterised by the presence of neoplastic cells, in single cells and clusters, that infiltrate all layers of nonneoplastic squamous epithelium. The differential diagnosis for EMPD includes melanoma in situ and secondary involvement of tumours from other sites, such as urothelial or cervical; pagetoid spread of tumor cells can also been seen at other sites, such as anorectal mucosa. The most frequently utilised biomarkers for confirming the diagnosis of EMPD include CK7 and GATA3; however, these biomarkers lack specificity. The purpose of this study was to evaluate TRPS1, a newly described breast biomarker, in pagetoid neoplasms of the vulva, scrotum and anorectum.Methods and results: Fifteen cases of primary EMPD of the vulva (two with associated invasive carcinoma) and four primary EMPD of the scrotum showed strong nuclear immunoreactivity for TRPS1. In contrast, five cases of vulvar melanoma in situ, one case of urothelial carcinoma with secondary pagetoid spread into the vulva and two anorectal adenocarcinomas with pagetoid spread into anal skin (one with associated invasive carcinoma) were negative for TRPS1. Additionally, weak nuclear TRPS1 staining was observed in nonneoplastic tissues (e.g. keratinocytes), but always with less intensity when compared to tumour cells. Conclusions: These results demonstrate that TRPS1 is a sensitive and specific biomarker for EMPD, and may be especially useful for excluding secondary involvement of the vulva by urothelial and anorectal carcinomas.