A diffuser for a multiway valve, in particular for a two-way valve, includes a tip (2) to be secured to the portion of the valve stem that projects above the cup. To withdraw a single product, the tip includes a cylindrical portion having a circular cross-section with as many transfer channels (22a, 22b) are there are pathways in the multiway valve, for sealingly connecting each transfer channel to the outlet of the corresponding pathway of the valve, each transfer channel opening outside the cylindrical portion of the tip in a different radial plane and in a different angular sector of the cylindrical portion. The diffuser is also provided with a pivotable cap (3) with a blind hole (31) having a shape complementary to the cylindrical portion of the tip, the pivotable cap having as many outlet channels (32a, 32b) as there are pathways in the multiway valve.