This study used the segmental long-bone defect model to assess the effects of osteoporosis on the formation of new bones and the osteoinductivity of recombinant human bone morphogenetic protein-2 (rhBMP-2). Seventy-two female Sprague-Dawley rats were divided into two groups: an osteoporosis group with ovariectomies and dexamathasone intramuscular injections and a sham group. When they reached 22 weeks in age, each group was further divided into two groups and a 5-mm defect was made in both fibular mid-shafts of each rat. One fibula in each rat was picked randomly and was injected with 0.05 mL of hydrogel carrier; the opposite fibula was injected with the same carrier mixed with rhBMP-2 (10 μg). After rearing for a further 5 and 9 weeks, the ratios of the lengths of the newly formed bones in the fibular defects were determined using micro-CT and undecalcified histology. The sham rhBMP-2-injected group-in all of the 5- and 9-week-kept groups-showed a significantly higher bridging bone formation ratio than the other three groups. The osteoporosis rhBMP-2-injected group showed a significantly higher ratio than both the non-rhBMP-2-injected sham hydrogel and the osteoporosis hydrogel groups. The comparison of the micro-CT parameters of the newly formed bones showed that the sham rhBMP-2 group at both 5 and 9 weeks compared with the osteoporosis rhBMP-2 group had significantly higher percentage bone volumes, trabecular thicknesses, and trabecular numbers, in addition to significantly lower specific surfaces, trabecular pattern factors, and structural model indices. The histology results showed that the sham-rhBMP-2 group began forming bridging bones in the defect areas at 5 weeks, and at 9 weeks, trabeculae and marrow spaces were observed. However, the osteoporosis rhBMP-2 group exhibited a relatively minor level of new bone and trabecula formation. Consequently, the rhBMP-2 group showed significantly increased bone formation in the osteoporosis rat fibular defect model compared with the hydrogel group, whereas the new bone quantities, qualities, and remodeling in the osteoporosis rhBMP-2 group were less effective than those in the sham-rhBMP-2 group, signaling that ovariectomy and corticosteroid-induced osteoporosis significantly undermines rhBMP-2 osteoinductivity.