The present invention relates to a device for monitoring a patient access, in particular a vascular access in extracorporeal blood treatment. The device is used to establish a substantially parallel connection between the arterial and venous tubes, or the arterial and venous puncture needles. If the venous needle slips out of the vascular access, the arterial cannula is forcibly withdrawn. This incident is reliably detected by the known protection systems which monitor the pressure in the arterial tube and/or an air intake. In addition, the present invention relates to an extracorporeal blood treatment device and to a method for monitoring a patient access, wherein the venous and arterial tubes or the puncture needles are connected to each other.