This paper investigates the fatigue upgrading of typical cover plate joint connections in existing steel bridge girders through cover plate extension. The upgraded detail is created by removing the existing transverse end weld, then welding the extension to the existing cover plate with the underlying plate acting as backing. The extended cover plate should end in a zone of low stress variation. The fatigue resistance of this detail is investigated by means of scaled specimen and beam testing as well as a corroborating detailed finite-element parametric study. The different potential crack initiation sites as well as the three-dimensional effects due to the presence of longitudinal welds are considered. It is found that, depending on the exact geometry of the joint, cracks may initiate either from the weld toe or the weld root. A classification of the joint in appropriate detail categories is proposed. Depending on the thicknesses of the cover plate extension, the existing cover plate, and the underlying plate, the detail category may vary between FAT 50 (or E') and FAT 80 (or D) according to standard classifications.