Objectives: Osteoporosis (OP) is a prevalent bone disease characterized by reduced bone mass and increased fracture risk, in part due to impaired osteogenic differentiation of bone marrow–derived mesenchymal stromal cells (BMSCs). Loss of AT-rich interactive domain–containing protein 2 (ARID2) attenuates BMSC osteogenesis, but the underlying mechanism remains to be elucidated. Methods: Human BMSCs were induced to undergo osteogenic or adipogenic differentiation. Lentiviral transduction was used to silence ARID2 and/or overexpress serine/threonine kinase 39 (STK39). Effects on BMSC fate were assessed by quantitative reverse-transcription PCR (qRT-PCR), MTT assay, alkaline phosphatase (ALP) staining, Alizarin Red S (ARS) staining, Oil Red O staining, and western blotting. Results: ARID2 expression increased over time during osteogenic induction. ARID2 silencing reduced BMSC proliferation and osteogenic differentiation while favoring adipogenesis. Conversely, STK39 overexpression enhanced proliferation and osteogenic differentiation and attenuated nuclear factor κB (NF-κB) signaling during osteogenic induction; these effects were reversed by ARID2 silencing. Conclusion: The STK39–ARID2 axis promotes osteogenic differentiation and suppresses adipogenic differentiation of BMSCs, at least in part via inhibition of NF-κB signaling. These findings highlight a potential molecular target for therapeutic strategies in OP.