An exhaust-gas turbocharger (1) having a turbine housing (2) which has a turbine inlet (3) and a turbine outlet (4) and which connects a wastegate duct (5) between the turbine housing inlet (3) and the turbine housing outlet (4), which wastegate duct can be opened and closed via a shut-off element (6). The shut-off element is in the form of a piston (6) which is guided in a longitudinally displaceable manner in an interior space (14) of a guide (7), and the wastegate duct (5) opens into the interior space (14) transversely with respect to the longitudinal central line (L) of the guide (7).