Device for fastening a sensor on containers with a flexible container wall, in particular mixing bags, comprising a sensor support which is fitted with a sensor, bears against an inner wall of the container wall at least with a rear subsurface averted from the sensor, and is guided with a central piece through an opening in the container wall in which the sensor support is fixed on the container wall by a clamping part which can be connected to the central piece, in which the container wall is clamped between the rear subsurface of the sensor support and on a bearing surface, facing an outer wall of the container, of the clamping part, and in which the rear subsurface bears sealingly against the inner wall of the container wall. The sensor support can also have an electronic or optical transmit unit which is connected to the sensor, in which case the transmit unit communicates in a wireless fashion with a receive unit arranged outside the container.