GA is an important phytohormone that regulates root growth and secondary metabolism. GRAS family transcription factors (TFs) are the key regulators of GA signaling. Here, we found that SmGRAS5 was co-expressed in the root periderm with tanshinones in Salvia miltiorrhiza. Overexpression (OE) of SmGRAS5 increased tanshinones accumulation and upregulated the biosynthetic genes. Antisense expression (AE) of SmGRAS5 reduced tanshinones accumulation and downregulated the biosynthetic genes. Yeast one-hybrid (Y1H), dual-luciferase (Dual-LUC), and electrophoretic mobility shift assays (EMSA) showed that SmGRAS5 promoted tanshinones biosynthesis by directly binding to the GARE motif in the promoter of SmKSL1 to induce its expression. However, overexpressing SmGRAS5 reduced GA content through downregulating the biosynthetic genes and also reduced root biomass. GA treatment further increased tanshinones accumulation and restored the root growth inhibited by overexpressing SmGRAS5. SmGRAS5 could not directly bind to the GA biosynthetic genes. Transcriptome analysis revealed the potential functions of SmGRAS5 in regulating secondary metabolism. Taken together, SmGRAS5 is involved in the regulation of GA-promoted tanshinones biosynthesis by directly activating the expression of SmKSL1, which suggests that SmGRAS5 may be a potential target for further metabolic engineering of tanshinones biosynthesis in S. miltiorrhiza.