In one aspect, a rotor blade assembly for a wind turbine may generally include a rotor blade extending lengthwise between a root and a tip. The rotor blade may include a pressure side and a suction side extending between a leading edge and a trailing edge. Additionally, the rotor blade assembly may include a chord extender having an attachment portion coupled to at least of the pressure side or the suction side and an extension portion extending outwardly from the attachment portion beyond the trailing edge. The extension portion may extend chordwise between a first end disposed adjacent to the trailing edge and a second end disposed opposite the first end. The extension portion may include a surface defined between the first and second ends. The extension portion may further include at least one stiffening rib projecting outwardly from the surface.