Seborrheic keratosis is a benign epidermal neoplasm, representing one of the most common skin tumors. Clonal seborrheic keratosis is one of the histological subtypes of this entity. It is an uncommon lesion which may resemble other benign or malignant lesion. We report a case of a 60-year-old woman presented with a 7 year history of a gradually growing, cutaneous lesion on her left arm. On physical examination, the lesion was elevated, well-circumscribed, measuring 5 cm in maximum diameter. The tumor was biopsied. Histopathological examination revealed the presence of well-defined nests of clear-looking or basaloid keratinocytes within an acanthotic epidermis, corresponding to the Borst-Jadassohn phenomenon. The tumor cells were small and monomorphic. We diagnosed this tumor as clonal seborrheic keratosis. Although surgical excision was recommended, our patient refused an operation. This case of clonal seborrheic keratosis is presented for its rarity and for differential diagnosis.