High glucose levels can induce mesangial cell proliferation and extracellular matrix (ECM) accumulation through the type I activin receptor-like kinase 5 (ALK5) signaling pathway. Salt-inducible kinase 1 (SIK1) prevents fibrosis by downregulating ALK5, while the expression level of the SIK1 protein itself is downregulated by glucose in neuronal cells following ischemia. In this study, we investigated the correlation between SIK1 and the ALK5 signaling pathway in a rat glomerular mesangial cell line (HBZY-1 cells). We found that high glucose levels downregulated the expression level of SIK1 and suppressed the phosphorylation of SIK1 at Thr-182. The downregulation of SIK1 by high glucose was accompanied by the activation of the ALK5 signaling pathway, while the overexpression of SIK1 in the HBZY-1 cells resulted in a decrease in the ALK5 protein level, as well in the levels of its downstream targets, including fibronectin and plasminogen activator inhibitor type I. In conclusion, high glucose may activate the ALK5 signaling pathway by downregulating SIK1, and SIK1 may be a protective factor against cellular proliferation and ECM accumulation in glomerular mesangial cells under high glucose conditions.