An opening device for opening a closed fluid container comprises a housing having a first chamber (1) for receiving the fluid container (3) and a second chamber (2) having an outlet (13). A part (6) of the fluid container that can be separated is arranged in the second chamber. A rotary drum (8) is rotatably supported in the second chamber. Said drum can be rotated from a starting position into a separating position in which it separates the separable container part from the fluid container. The rotational axis of the rotary drum is disposed at an angle to the connecting axis between the first and second chambers. A system for mixing at least two components uses such an opening device and a mixing container (18) having a connecting piece (19), to which the opening device is removably connected to establish a fluid connection.