A diffuser includes a cone body including an apex portion, a bottom portion, and a side edge portion. The apex portion forms a partial spherical surface, and the side edge portion is aspherical and is connected between the apex portion and the bottom portion. The apex portion satisfies: 2R/3≤r≤R, where r is a radius of curvature of the apex portion, and R is a radius of curvature of a spherical diaphragm of a tweeter speaker. A loudspeaker including a tweeter speaker and a diffuser is also provided.