© The Author(s) 2022.A 71-year-old woman was hospitalized with hematuria and underwent cystourethroscopy. Cystourethroscopy revealed a polypoid bladder tumor. Histopathologic examination showed complex villiform growth pattern, slit-like serrations, and ectopic crypts lined by epithelium with eosinophilic cytoplasm, pseudostratified elongated nuclei, consistent with traditional serrated adenoma. Nephrogenic and intestinal metaplasia with severe inflammation were present in adjacent bladder mucosa. Molecular study of the polyp revealed mutation (p.G12V) in codon 12 of exon 2 of the KRAS gene. Traditional serrated adenoma is a rare type of colonic serrated polyp, making up less than 1% of the colonic polyps with a predilection to distal colon. In the literature, there is no traditional serrated adenoma reported outside the gastrointestinal tract. Here in we report the first extra-gastrointestinal traditional serrated adenoma within the bladder and bladder diverticulum, arising from intestinal metaplasia. The present study reports an additional information on molecular background of this unusual bladder polyp.