The opening/closing device for a disposable flexible container includes a base sheet (1) having a first layer (1a) with a lower surface covered by a first adhesive (3), a second layer (1b) attached on the first layer (1a) by a second adhesive (8a), and contour cuts (5) defining an opening/closing flap (2) connected to the rest of the base sheet (1) by a fold line (6) that has aligned discontinuous cuts affecting both the first and second layers (1a, 1b) and the first and second adhesives (3, 8a). Optionally, the base sheet (1) includes a third layer (1c) attached on the second layer (1b) by a third adhesive (8b), and optionally the aligned discontinuous cuts of the fold line (6) also affect the third layer (1c).