Melatonin regulates broad aspects of plant responses to various biotic and abiotic stresses, but the upstream regulation of melatonin biosynthesis by these stresses remains largely unknown. Herein, we demonstrate that transcription factor heat-shock factor A1a (HsfA1a) conferred cadmium (Cd) tolerance to tomato plants, in part through its positive role in inducing melatonin biosynthesis under Cd stress. Analysis of leaf phenotype, chlorophyll content, and photosynthetic efficiency revealed that silencing of the HsfA1a gene decreased Cd tolerance, whereas its overexpression enhanced plant tolerance to Cd. HsfA1a-silenced plants exhibited reduced melatonin levels, and HsfA1a overexpression stimulated melatonin accumulation and the expression of the melatonin biosynthetic gene caffeic acid O-methyltransferase 1 (COMT1) under Cd stress. Both an in vitro electrophoretic mobility shift assay and in vivo chromatin immunoprecipitation coupled with qPCR analysis revealed that HsfA1a binds to the COMT1 gene promoter. Meanwhile, Cd stress induced the expression of heat-shock proteins (HSPs), which was compromised in HsfA1a-silenced plants and more robustly induced in HsfA1a-overexpressing plants under Cd stress. COMT1 silencing reduced HsfA1a-induced Cd tolerance and melatonin accumulation in HsfA1a-overexpressing plants. Additionally, the HsfA1a-induced expression of HSPs was partially compromised in COMT1-silenced wild-type or HsfA1a-overexpressing plants under Cd stress. These results demonstrate that HsfA1a confers Cd tolerance by activating transcription of the COMT1 gene and inducing accumulation of melatonin that partially upregulates expression of HSPs.