An implantable vessel fluid sensor is configured to sense at least one vessel fluid parameter of a vessel. The implantable vessel fluid sensor includes a tubular body having a first end portion. The first end portion is configured to be inserted into and to form a sealed junction with an open vessel end of the vessel. The implantable vessel fluid sensor further includes a sensor unit connected to the tubular body. The sensor unit includes a sensor region configured to be in direct contact with the vessel fluid in a sealed junction state. A minimum distance between the sensor region and the first end portion is at most 10 times an outer diameter of the first end portion of the tubular body.