Transforming growth factor‐beta1 (TGF‐β1), a highly abundant growth factor in skeletal tissues, stimulates matrix metalloproteinase‐13 (MMP‐13) expression in osteoblastic cells. MMP‐13 plays a critical role in bone remodeling. Runx2, a bone transcription factor, is required for TGF‐β1‐mediated stimulation of MMP‐13 expression in osteoblastic cells. In this study, the molecular mechanism responsible for TGF‐β1‐stimulation of MMP‐13 expression via Runx2 in osteoblastic cells was elucidated. TGF‐β1 stimulated the phosphorylation of Runx2 at serine amino acids, and ERK inhibition blocked this effect in rat (UMR106‐01) and human (MG‐63) osteoblastic cells. Pretreatment with okadaic acid, a serine‐threonine phosphatase inhibitor, increased Runx2 serine phosphorylation in osteoblastic cells. When cells were pretreated with an ERK inhibitor, TGF‐β1‐mediated stimulation of MMP‐13 mRNA expression decreased. Nano‐ESI/LC/MS analysis identified that TGF‐β1 stimulates Runx2 phosphorylation at three serine amino acids. Transient transfection of mouse mesenchymal stem cells (C3H10T1/2) with Runx2 serine mutant constructs decreased TGF‐β1‐mediated Runx2 serine phosphorylation. A luciferase reporter assay identified that TGF‐β1 stimulated MMP‐13 promoter activity in these cells only in the presence of the wild Runx2 construct, and not with mutant Runx2. Thus, TGF‐β1 stimulates the phosphorylation of Runx2 at three serine amino acids, and this event is required for MMP‐13 expression in osteoblastic cells. Hence, this study contributes to the knowledge of events governing bone remodeling and bone‐related diseases. A schematic diagram representing the TGF‐β1‐induced stimulation of ERK‐dependent Runx2 phosphorylation, with the corresponding sites, required for MMP‐13 promoter activation in human osteoblast cells.