The present invention provides a device and a method for monitoring the bladder volume of a subject. A device for monitoring the bladder volume of a subject comprises a sensor to be attached to a region on the exterior surface of the abdomen of the subject, the region corresponding to the bladder of the subject, the sensor being configured to obtain a sensor signal indicating the bladder volume of the subject; a controlling unit configured to generate a control action signal if it determines, based on the sensor signal, that the change of the bladder volume of the subject exceeds a predetermined amount; an ultrasound probe to be attached to the subject and configured to emit, in response to the control signal, an ultrasonic signal toward the bladder of the subject and receive echo signals from the bladder of the subject; a deriving unit configured to derive the bladder volume of the subject from the received echo signals. According to the device, long-time monitoring of the bladder volume of the subject may be achieved.