A spout for a spill-proof beverage container has a side wall defining an outer surface and an inner surface defining a discharge passage. A valve in the discharge passage includes two flanks formed integrally with the side wall and projecting from opposite sides of the inner surface of the spout. The front end faces of the flanks mate with one another along a slit that extends generally parallel to the longer axis of the spout. At least the front portions of the flanks are inclined away from the mouth of the spout such that pressure within the container acts to urge the flanks against one another so as to maintain the slit closed. Deformation of the side wall when the spout is held between the lips of a drinker acts to open the slit and create an opening between the flanks to allow the beverage to be discharged.