Melatonin (MT) is a low molecular weight compound with a wide variety of biological functions in plants. It is known to delay senescence in various fruits and vegetables. This study demonstrates that MT (100 μmol/L) delayed the senescence of pak choi stored at 20 °C. A total of 1121 differentially expressed genes (DEGs) were identified in an MT-treated group in comparison to untreated pak choi. Analysis of DEGs revealed that MT-induced delay in senescence involved reduction in the expression of chlorophyll metabolism genes and regulation of the expression of hormone-related genes and leaf senescence-related transcription factors. MT decreased the activity of chlorophyll metabolism-related enzymes Mg-dechelatase, pheophytinase, and pheide a oxygenase, decreased the concentrations of abscisic acid and jasmonic acid, and increased auxin concentration. MT delayed the impact of senescence-related transcription factors BcNAC41 and BcNAC87 on tobacco leaf senescence. This study provides an insight into the molecular mechanisms of MT-induced delay of postharvest senescence in pak choi and confirms its potential as a preservation technique.