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Chilling causes a significant decline in photosynthesis in tomato plants. Tomato tryptophan decarboxylase gene 1 (<i>SlTDC1</i>) is the first rate-limiting gene for melatonin (MT) biosynthesis and is involved in the regulation of photosynthesis under various abiotic stresses. However, it is not clear whether <i>SlTDC1</i> participates in the photosynthesis of tomato under chilling stress. Here, we obtained <i>SlTDC1</i> overexpression transgenic tomato seedlings, which showed higher <i>SlTDC1</i> mRNA abundance and MT content compared with the wild type (WT). The results showed that the overexpression of <i>SlTDC1</i> obviously alleviated the chilling damage to seedlings in terms of the lower electrolyte leakage rate and hydrogen peroxide content, compared with the WT after 2 d of chilling stress. Moreover, the overexpression of <i>SlTDC1</i> notably increased photosynthesis under chilling stress, which was related to the higher chlorophyll content, normal chloroplast structure, and higher mRNA abundance and protein level of Rubisco and RCA, as well as the higher carbon metabolic capacity, compared to the WT. In addition, we found that <i>SlTDC1</i>-overexpressing seedlings showed higher W_k (damage degree of OEC on the PSII donor side), φ_Eo (quantum yield for electron transport in the PSII reaction center), and PI_ABS (photosynthetic performance index) than WT seedlings after low-temperature stress, implying that the overexpression of <i>SlTDC1</i> decreased the damage to the reaction center and donor-side and receptor-side electron transport of PSII and promoted PSI activity, as well as energy absorption and distribution, to relieve the photoinhibition induced by chilling stress. Our results support the notion that <i>SlTDC1</i> plays a vital role in the regulation of photosynthesis under chilling stress.