A boot for a constant velocity universal joint includes large- and small-diameter mounting portions to be mounted to an outer joint member and a shaft, respectively, with boot clamps, a bellows portion integrally coupling the large- and small-diameter mounting portions to each other, and a cylindrical shoulder portion formed at a bellows portion-side end of the large-diameter-side mounting portion, and connected to the bellows portion through two auxiliary crest portions and one auxiliary trough portion between the auxiliary crest portions. A radial distance between each of the auxiliary crest portions and the auxiliary trough portion is shorter than a radial distance between a crest portion and a trough portion. A diameter passing through a center of the radial distance between each of the auxiliary crest portions and the auxiliary trough portion is larger than a diameter of the cylindrical shoulder portion.