Abstract: An adapter for receiving a rectangular beverage container such as a "juice box" or the like and supporting the same stably within a cylindrical recess or circular aperture of a beverage holder comprises a cylindrical lower section for resting within the beverage holder, a transition section, and a rectangular upper section including a rectangular recess for receiving the rectangular beverage container. The adapter may be inexpensively molded as a tubular member of plastic material. The adapter may be supplied in connection with a cap adapted to be releasably affixed to the adapter, to provide a squeeze-preventing shell for a juice box or the like.