Periodontium-derived stem cells (pdSCs) can be cultured as dentospheres and differentiated into various cells of the neuronal lineage such as glial cells, thereby demonstrating their stem cell state. This study investigated whether pdSCs could be differentiated into the osteogenic lineage and, if so, whether these cells are able to regenerate periodontal tissue in vivo in an athymic rat model. Human adult pdSCs were isolated during minimally invasive periodontal surgery and expanded in vitro. To induce osteogenic differentiation, expanded pdSCs were cultured for 3 weeks in osteogenic differentiation media. Staining for alkaline phosphatase expression was positive, suggesting osteogenic differentiation. For in vivo studies, pdSCs were delivered onto suitable collagen sponges and implanted into periodontal defects on the right buccal cortex of the mandible in 16 immunodeficient nude rats. Histologic analysis of samples from the test side revealed reformation of periodontal ligament-like tissue, collagen fibers, and elements of bone, but no functional periodontal tissue regeneration. The data show that human adult pdSCs are capable of regenerating elements of bone and collagen fibers in an in vivo animal model.