A photoresist-removing composition includes a polar organic solvent, an alkyl ammonium hydroxide, an aliphatic amine not including a hydroxy group, and a monovalent alcohol. To manufacture a semiconductor device, a photoresist pattern may be formed on a substrate, and the photoresist-removing composition may then be applied to the photoresist pattern. To manufacture a semiconductor package, a photoresist pattern including a plurality of via holes may be formed on a substrate. A plurality of conductive posts including a metal may be formed inside the plurality of via holes, and the photoresist pattern may be removed by applying a photoresist-removing composition of the inventive concept to the photoresist pattern. A semiconductor chip may be adhered to the substrate between the respective conductive posts.