To investigate the anticancer and osteogenic properties and mechanism of action of a selenium (Se4+) hydroxyapatite (HA) nanodelivery system for osteosarcoma (OS) therapy. We loaded different concentrations of Na2SeO3 (3% and 6%) into HA nanoparticles (HANP) for OS therapy. Simultaneously, we tested the stability and pH responsiveness of Se-HANP in vitro experiments, and OS MG-63 cells and mice BMSCs were cultured to further evaluate its permeability, anticancer ability and mineralization ability [alkaline phosphatase (ALP), osteocalcin (OCN)]. In addition, We conducted in vivo experiments by constructing OS mouse models to further investigate the anticancer mechanism of Se-HANP. Se-HANP showed good stability in blood; an acidic environment (pH 5) was more conducive to Se4+ ion release, and could effectively penetrate into OS MG-63 cells. In cell experiments, compared with HANP, Se-HANP could more effectively improve BMSCs viability and enhanced their mineralization performance, such as improving ALP and OCN levels. In addition, Se-HANP could effectively inhibit the activity and invasiveness of MG-63 cells; 6% Se-HANP showed stronger anticancer effects. In OS in vivo mouse experiments, Se-HANP could effectively inhibit tumor tissue growth (volume and weight) in a concentration dependent manner. In addition, Se4+ activated endogenous and exogenous apoptotic pathways by increasing the level of reactive oxygen species (8-OHdG) in tumor cells, leading to tumor cell apoptosis. Se-HANP could effectively penetrate OS cells, and its good stability and pH responsiveness could improve the anticancer efficiency of Se4+. Moreover, its excellent mineralization ability could effectively repair the bone defects caused by OS. Accordingly, Se-HANP can have great potential in OS treatment.