Melatonin (N-acetyl-5-methoxytryptamine) functions as a ubiquitous modulator in multiple plant developmental processes and various stress responses. However, the involvement of melatonin in natural leaf senescence and the underlying molecular mechanism in Arabidopsis remain unclear. In this study, we found that the endogenous melatonin level was significantly induced in a developmental stage-dependent manner, and exogenous melatonin treatment delayed natural leaf senescence in Arabidopsis. The expression level of AUXIN RESISTANT 3 (AXR3)/INDOLE-3-ACETIC ACID INDUCIBLE 17 (IAA17) was significantly downregulated by exogenous melatonin treatment and decreased with developmental age in Arabidopsis. Further investigation indicated that AtIAA17-overexpressing plants showed early leaf senescence with lower chlorophyll content in rosette leaves compared with wild-type plants, while AtIAA17 knockout mutants displayed delayed leaf senescence with higher chlorophyll content. Notably, exogenous melatonin-delayed leaf senescence was largely alleviated in AtIAA17-overexpressing plants, and AtIAA17-activated senescence-related SENESCENCE 4 (SEN4) and SENESCENCE-ASSOCIATED GENE 12 (SAG12) transcripts might have contributed to the process of natural leaf senescence. Taken together, the results indicate that AtIAA17 is a positive modulator of natural leaf senescence and provides direct link between melatonin and AtIAA17 in the process of natural leaf senescence in Arabidopsis.