The invention relates to a drain valve (1) for a sanitary cistern, in particular a concealed cistern, having a valve housing (2) with a drain opening (4), the drain opening (4) being delimited by a valve seat (6), and a valve body (7), which rests in a sealing manner on the valve seat (6) in the closed state of the drain valve, the valve housing (2) having a housing chamber (11), arranged upstream of the valve seat (6) in the direction of flow, for receiving water. To allow the drain valve to be arranged outside the cistern and to enable access to the drain valve for maintenance as well as removal via an inspection opening that can be covered by a toilet bowl or urinal bowl, the invention suggests that
the valve body (7) is provided with an auxiliary chamber (12) for receiving water, the receiving volume of which is smaller for water in the open state of the drain valve (1) than in the closed state of the drain valve (1), the auxiliary chamber (12) being delimited by a flexible and/or movable wall section, and wherein the auxiliary chamber (12) is associated with an auxiliary drain valve (16) which in a first position permits an inflow of water into the auxiliary chamber (12) and in a second position permits an outflow of water out of the auxiliary chamber (12).