Congenital smooth muscle hamartoma usually manifests as a well-circumscribed, hyperpigmented plaque, frequently hypertrichotic, on the trunk or extremities. We report such a lesion in a 7-month-old girl that presented as a linear, mottled, purplish red plaque appearing in areas to be atrophic, involving her right buttock, posterior thigh and leg, and fifth toe. Although the clinical appearance suggested linear morphea, a biopsy specimen had numerous haphazardly oriented bundles of smooth muscle in the reticular dermis. Masson trichrome staining, smooth muscle specific actin, and electron microscopic studies confirmed the smooth muscle nature of the cells. A diagnosis of smooth muscle hamartoma was made. To our knowledge, this linear clinical presentation has not been described previously.