A rotor blade for a wind turbine includes a plurality of individual blade segments, with each blade segment defining an internal passage extending between the longitudinal ends of the blade segment. A rigid spar member extends longitudinally through the internal passages of the individual blade segments such that the blade segments are aligned and connected end-to-end on said spar member to define a complete rotor blade from a root to a blade tip. The spar member has a cross-sectional profile that is keyed to the cross-sectional profile of the internal passage in the blade segments. The spar member includes oppositely facing spar caps that engage against the inside surfaces of the blade segments within the internal passages.