Using exogenous melatonin, melatonin-deficient tomato plants with a silenced caffeic acid <i>O</i>-methyltransferase (<i>COMT</i>) gene, and <i>COMT</i>-overexpressing plants with cosuppression of sulfate transporter (<i>SUT)1</i> and <i>SUT2</i> genes, we found that melatonin deficiency decreased S accumulation and aggravated Cd phytotoxicity, whereas exogenous melatonin or overexpression of <i>COMT</i> increased S uptake and assimilation, resulting in an improved plant growth and Cd tolerance.