Amino acids are utilized as medical raw materials and food additives. It is well-known that most of the amino acids show polymorphism. Differences in the crystal structures affect the physical and chemical properties. Therefore, an effective method for polymorph control is required in the crystallization process. Polymorph control of L-arginine hydrochloride (L-ArgHCl) on antisolvent crystallization was achieved by ultrasonic irradiation. L-ArgHCl exhibits polymorphism and crystallizes into three distinct crystal forms. Without irradiation, the metastable form was selectively crystallized. In contrast, the unstable form was selectively crystallized by irradiation. Irradiation after the addition of an antisolvent led to a shorter induction time.