In a method and a device for testing a weld seam (8), located on the inner surface of a reactor pressure vessel (2), by which the outer circumference of an instrumentation nozzle (4) leading into the interior of this reactor pressure vessel (2) is welded onto the reactor pressure vessel (2), an ultrasound test probe (20) with a linear ultrasound transducer array (30) is inserted into the instrumentation nozzle (4), which ultrasound transducer array (30) is parallel to the central axis (12) of the instrumentation nozzle (4) in terms of its longitudinal direction, and is used to couple a transmitted ultrasound signal (S) into the instrumentation nozzle (4) in the region of the weld seam (8) and to receive a reflected ultrasound signal (R).