Harlequin syndrome (HS) is a rare disorder of the sympathetic nervous system which presents with unilateral decreased sweating and flushing of the face, neck, and chest in response to heat, exercise, or emotional factors. The contralateral side displays a compensatory overreaction to provide normal heat regulation of the face as a whole. In the literature, most of the cases are primary in nature and no underlying cause could be identified. Harlequin sign is used to denote these symptoms in patients who also exhibit associated oculosympathetic paresis, such as Horner syndrome, Adie syndrome, and Ross syndrome.We report a rare case of a 13-year-old boy who presented with complaints of flushing and sweating of the left side of the face after exertion, while the right side remained dry and maintained its normal color. No structural abnormality was identified on detailed work up. Thus, diagnosis of classic idiopathic HS was made. Despite the rarity of this syndrome, dermatologists should be acquainted with this distinctive entity and should refer the patient for complete ophthalmological and neurological examination.