The invention relates to a valve mechanism (1; 1′) for controlling a fluid, comprising a housing (2; 2′) that has a fluid duct (3), further comprising a valve body (5) that is mounted on the housing (2) so as to be adjustable between an open position in which the valve body allows the fluid (F) to flow through the fluid duct (3) and a closed position in which the valve body fluid-tightly seals the fluid duct, and comprising a membrane-like sealing element (13) which is secured to the housing (2) at a first end portion (14) and to the valve body (5) at a second end portion. In the open state of the valve body (5), a side of the sealing element (13) that faces away from the fluid duct (3) rests against a wall section (17) between the two end portions (14, 15).