Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease (ESRD), with mitochondrial dysfunction playing a crucial role in its progression. Sirtuin 3 (SIRT3) is an important mitochondrial deacetylase that maintains mitochondrial homeostasis and represents a promising therapeutic target for DKD. In this study, we developed a novel allosteric activator of SIRT3, SZC-6, a 3-aryl coumarin derivative, which exhibits stronger activation of SIRT3 compared to the currently most effective agonist, C12(7-Hydroxy-3-(4-methoxyphenyl)-2H-benzopyran-2-one). We found that SZC-6 directly binds to SIRT3 and enhances its deacetylating activity on SIRT3 and renal tubular epithelial cell proteins in a dose-dependent manner. Single-cell transcriptome analysis of DKD patient data from the GEO database suggested that renal tubular epithelial cells may be the primary cell population affected by SZC-6. Additionally, SZC-6 reduces age-induced fibrosis in renal tubular epithelial cells and improves renal function and fibrosis in diabetic mice, while these effects are abolished in SIRT3 knockout mice. Furthermore, SZC-6 increases the expression of MnSOD (SOD2), reduces reactive oxygen species (ROS) and mitochondrial ROS (mtROS) levels, and enhances mitochondrial membrane potential, respiratory function, and ATP synthesis in age-treated renal tubular cells. SZC-6 also alleviates mitochondrial fragmentation, decreases Drp1 expression, and increases Mfn2 expression. A co-analysis of SIRT3 substrates and the MnSOD promoter revealed that Foxo3a is a key regulator. Mechanistically, SZC-6 enhances the deacetylation activity of SIRT3 by reducing the acetylation level at the Lys271 site of Foxo3a, leading to the accumulation of Foxo3a in the nucleus. Activated Foxo3a upregulates MnSOD transcription and restores mitochondrial function. Inhibition of Foxo3a negates the effects of SZC-6. In summary, SZC-6 significantly alleviates the pathological damage of DKD by activating the SIRT3-Foxo3a axis, providing a potential new strategy for SIRT3 activators in the treatment of DKD.