Melatonin (MT) is a low molecular weight compound with multiple biological functions in plants. It is known to delay leaf senescence in various species. However, no data are available on the MT signaling pathway in postharvest vegetables. This study demonstrates that MT increases cGMP concentration and the expression of the cGMP synthesis gene BcGC1 in pak choi. The cGMP inhibitor LY83583 destroys effect of MT delaying the leaf senescence. LY83583 also prevents MT treatment from reducing the expression of chlorophyll metabolism-related genes (BcNYC1, BcNOL, BcPPH1/2, BcSGR1/2, and BcPAO) and senescence genes (BcSAG12 and BcSAG21). It also inhibits MT from reducing the activity of the key chlorophyll catabolism enzymes Mg-dechelatase, pheophytinase, and pheide a oxygenase. Thus, the ability of MT to maintain high levels of chlorophyll metabolites is also destroyed. The Arabidopsis cGMP synthetic gene mutant atgc1 was used to confirm that delayed leaf senescence caused by MT is mediated, at least in part, by the second messenger cGMP.