Periodontitis is featured as the periodontium's pathologic destruction caused by the host's overwhelmed inflammation. Omentin-1 has been reported to be aberrantly downregulated in patients with periodontitis, but the specific regulation of Omentin-1 during the pathogenesis of periodontitis remains unclear. In this study, human periodontal ligament stem cells (hPDLSCs) were stimulated by lipopolysaccharide (LPS) from Porphyromonas gingivalis to establish an in vitro inflammatory periodontitis model. hPDLSCs were treated with recombinant human Omentin-1 (250, 500 and 750 ng/mL) for 3 h before LPS stimulation. Results revealed that Omentin-1 significantly inhibited LPS-induced inflammation in hPDLSCs through reducing the production of proinflammatory cytokines (tumor necrosis factor-α (TNF-α), interleukin (IL)-1β and IL-6) and downregulating the expression of Cox2 and iNOS. Meanwhile, Omentin-1 significantly enhanced alkaline phosphatase (ALP) activity and Alizarin red-stained area, accompanied by increasing expression osteogenic markers BMP2, OCN and Runx2, confirming that Omentin-1 restores osteogenic differentiation in LPS-induced hPDLSCs. In addition, the conditioned medium (CM) from LPS-induced hPDLSCs was harvested to culture macrophages, which resulted in macrophage polarization towards M1, while CM from Omentin-1-treated hPDLSCs reduced M1 macrophages polarization and elevated M2 polarization. Furthermore, Omentin-1 also inhibited LPS-triggered endoplasmic reticulum (ER) stress in hPDLSCs, and additional treatment of the ER stress activator tunicamycin (TM) partially reversed the functions of Omentin-1 on inflammation, osteogenic differentiation and macrophages polarization. In summary, Omentin-1 exerted a protective role against periodontitis through inhibiting inflammation and enhancing osteogenic differentiation of hPDLSCs, providing a novelty treatment option for periodontitis.