A ball valve (1) has a blocking element (3) movably guided in a fluid passage of a valve housing (2) and movable by an actuating device (4) from a pass-through position connecting at least two connecting points (5) to one another in the valve housing (2) into a blocking position interrupting this connection, and vice versa. A sealing device has at least one sealing ring which, together with its central opening, comprises the fluid passage (6) and seals the fluid passage along at least one sealing gap. The ring profile of the sealing ring has a ring opening that interrupts this profile such that the sealing ring changes from an initial shape to an installation and different shape that seals the sealing gap with a predefinable pretension having an increased sealing force effect.