Osteoporosis is a major global health issue in elderly people. Because Wnt/β-catenin signaling plays a key role in bone homeostasis, we screened activators of this pathway through cell-based screening, and investigated indirubin-3′-oxime (I3O), one of the positive compounds known to inhibit GSK3β, as a potential anti-osteoporotic agent. Here, we show that I3O activated Wnt/β-catenin signaling via inhibition of the interaction of GSK3β with β-catenin, and induced osteoblast differentiation in vitro and increased calvarial bone thickness ex vivo. Intraperitoneal injection of I3O increased bone mass and improved microarchitecture in normal mice and reversed bone loss in an ovariectomized mouse model of age-related osteoporosis. I3O also increased thickness and area of cortical bone, indicating improved bone strength. Enhanced bone mass and strength correlated with activated Wnt/β-catenin signaling, as shown by histological analyses of both trabecular and cortical bones. I3O also restored mass and density of bone in hindlimb-unloaded mice compared with control, suspended mice, demonstrating bone-restoration effects of I3O in non-aged–related osteoporosis as well. Overall, I3O, a pharmacologically active small molecule, could be a potential therapeutic agent for the treatment and prevention of osteoporosis. © 2014 American Society for Bone and Mineral Research.