The invention relates to a protective device (10) for a drainage valve (27) fitted to bags or “bags in boxes” intended for the transport and handling of liquid or quasi-liquid substances, said device (10) being formed by a sheet (1) of flexible material folded in on itself, two parallel side edges of said sheet being fixed to one another over at least a part of their length so as to form an approximately rectangular envelope which is open on one of its sides, called the insertion side, into which a reinforcing panel (30) located on the side of the bag and provided with the drainage valve (27) is inserted, said panel (30) having a thickness e1 and being provided with a hole (T) for said drainage valve (27) to pass through. The envelope has at least one opening (4) on its front and/or rear side for the drainage valve (27) to pass through, and at least one closed compartment (5A, 5B, 5C) containing flexible material, said compartment causing an overthickness e2 at the surface of the envelope, such that the sum of e1 and e2 is approximately equal to or greater than the height H of the drainage valve (27).