A vacuum valve comprises a valve housing which has an inlet opening and an outlet opening penetrating opposite walls of the valve housing, a valve seat surrounding the inlet opening, and a closure unit which is arranged in an interior space of the valve housing and which has a valve plate which is adjustable between an open position in which it releases the inlet opening, an intermediate position in which it covers the inlet opening but is lifted from the valve seat, and a closed position in which it covers the inlet opening and is pressed against the valve seat, and at least one adjusting piston which is arranged in an interior space of the closure unit and, for the purpose of adjusting the valve plate between its intermediate position and its closed position, is displaceable in direction of its longitudinal center axis between a passive position in which the valve plate is in its intermediate position and an active position in which the valve plate is in its closed position. At least one work space of the closure unit that can be acted upon by pressure gas for displacing the at least one adjusting piston into its active position is bounded by a diaphragm seal. At least one tappet is arranged at the adjusting piston and is guided out of the interior space of the closure unit through an opening and is supported directly or indirectly at the valve housing when the adjusting piston is displaced from its passive position into its active position.