An ultrasonic knife has a cutting portion formed with at least one cutting edge and a fastening portion. The fastening portion has an external thread for connection to a sonotrode. The cutting portion and the fastening portion are formed in one piece from cemented carbide material. The cemented carbide material includes hard particles that are, at least predominantly, formed by tungsten carbide and a metallic binder. The external thread of the fastening portion has a thread pitch p and is shaped such that the root of the thread turns has a rounded shape with a root radius R of 0.2*p≤R≤0.3*p.