Linear porokeratosis is a rare variant of porokeratosis that most often presents in newborns and children; development of this porokeratosis variant in adulthood is far less common. We report the case of a 25-year-old female who presented with a progressive eruption on the proximal upper extremity of 6-year duration, which was ultimately diagnosed as adult-onset linear porokeratosis and safely treated with oral isotretinoin. We propose that a sporadic mutation resulting in mosaicism after birth may explain the development of linear porokeratosis in adulthood, although the exact trigger of such a somatic mutation is not known. This case also describes a unique clinical presentation, with linear porokeratosis lesions originating on the proximal extremity rather than on the more common distal extremity. This demonstrates a distinctive clinical presentation not seen in the pediatric forms of disease.