Abstract: Guide which allows a precise patellar cut to be made for fitting the prosthetic patella of a knee prosthesis using a trial femoral component, having a sliding plate (20) of a pin guide (30), a pin guide (30) comprising at least two channels (33) for guiding pins (50), which are installed perpendicularly to the sagittal plane of the condyle, and comprising two panels having two surfaces (32, 34) arranged at a right angle, the first panel having a flat surface (32) and a central aperture (31), and the second panel having a flat surface (34) towards the inside of the guide, a securing element (40) which cooperates with the shaft (23) to lock the pin guide (30) with respect to the sliding plate (20) and thus avoid translation movements.