Steroid-induced osteonecrosis of the femoral head (SONFH) is a rapidly progressing and disabling complication of long-term glucocorticoid therapy, lacking effective early-stage intervention mechanisms. Its early manifestation involves a fate shift in bone marrow mesenchymal stem cells (BMSCs) characterized by decreased osteogenic differentiation (OGD) and increased adipogenic differentiation (AGD), yet the upstream regulatory mechanisms remain unclear. Herein, we integrated AGD-related microRNA (miRNA) microarray data with exosomal miRNA sequencing data and identified miR-199a-3p as a crucial candidate driver of this lineage imbalance. Our results revealed the up-regulation of miR-199a-3p in SONFH tissues and in glucocorticoid-treated cellular models, and indicated that its overexpression suppresses the OGD of BMSCs while markedly promoting the AGD. Further integrating mRNA-sequencing profiling during AGD with target prediction, protein–protein interaction network analysis, and dual-luciferase reporter assays, we confirmed integrin β8 (ITGB8) as a direct target of miR-199a-3p, which is consistently decreased in SONFH tissues and during adipogenic induction. We further revealed that miR-199a-3p suppressed the OGD of BMSCs by repressing ITGB8 expression, thereby inactivating the focal adhesion kinase (FAK)–extracellular signal-regulated kinase (ERK)–runt-related transcription factor 2 (RUNX2) signaling cascade. Conversely, silencing miR-199a-3p restores ITGB8 levels, reactivates this pathway, and corrects the OGD/AGD bias. In vivo, local administration of antagomiR-199a-3p in a SONFH rat model markedly improved trabecular bone architecture, increased bone mass, and up-regulated RUNX2 expression. These findings reveal for the first time that the miR-199a-3p/ITGB8–FAK–ERK–RUNX2 axis represents an unrecognized pathogenic pathway in SONFH, and support the local suppression of miR-199a-3p as a translatable early intervention strategy.