Background <p>Osteosarcoma (OS) is a highly malignant tumor arising from mesenchymal tissues and manifests with bone and joint pain and a local mass. OS patients typically have a poor prognosis and no significant improvement has been achieved over the last few decades. Isoforsythiaside (IFA), a monomer compound originating from a commonly-used Chinese medicinal herb named Fructus Forsythiae, recently was reported to show potential anti-tumor activity in breast tumors. However, the role of IFA in OS progression and therapy, especially in immune therapy, remains largely unexplored.</p> Methods <p>Cell proliferation, migration, and colony formation abilities of OS cells were analyzed using CCK-8 detection, wound-healing assay, colony formation assay, and nude mouse xenotransplantation models. Immunohistochemical staining (IHC) was utilized to assess the expression level of Ki67 and the number of CD8⁺ T cells in tumor tissue. In addition, quantitative real-time polymerase chain reaction (qPCR) and cell transfection were performed to explore the mechanism of IFA anti-tumor action.</p> Results <p>The results of the in vitro and in vivo experiments revealed that IFA inhibited growth, migration, and invasion in OS cells. Notably, histone lysine-specific demethylase 1 (LSD1, also named KDM1A) downregulation was found to play a vital role in IFA inhibition on OS progression. Finally, the in vivo experimental results demonstrated that IFA promoted CD8⁺ T cell infiltration and enhanced anti-PD1 therapeutic effect by downregulating LSD1.</p> Conclusions <p>Our findings in this study suggested that downregulating LSD1 exhibited remarkable activity in inhibiting OS progression and enhanced anti-PD1 therapeutic effect. The combined application of IFA and anti-PD1 may be a promising treatment strategy for OS.</p>