A valve assembly for use with a self-sealing collapsible storage bag, the valve assembly having a first portion comprising a housing that has a sealing surface and contains a valve mechanism. A second portion has a sealing surface that is configured to mate with the sealing surface of the first portion. A snap mechanism is configured to allow the first and second portions to be snapped together with their respective sealing surfaces facing each other to form a substantially airtight seal. In operation, the first and second portions are positioned on opposing sides of a panel of a plastic storage bag, and snapped together so as to sandwich the panel between the first and second portions of the valve assembly. The plastic bag is pierced in the area of the valve assembly either during installation of the valve assembly, or later by insertion of a nozzle. Gas or fluid such as air may be selectively removed from or added to the interior of the storage bag through the valve mechanism.