SlTDC1, a candidate gene for tryptophan decarboxylase (TDC) in tomato, was the focus of this study because SlTDC1 may play a role in the biosynthesis of serotonin (Hano et al., 2017), which is a novel functional ingredient because of its anti-obesity effects; further, its developmental roles are largely unknown. Tomato was transformed with SlTDC1 (TDX lines) to demonstrate its enzymatic function, the developmental roles of SlTDC1 and serotonin, and the possibility of molecular breeding. Transformation increased serotonin concentration three times or more in fruit without growth inhibition; in contrast, the concentration of tryptophan, which is the substrate of serotonin biosynthesis, decreased. The results showed the roles of SlTDC1 and its usefulness in producing serotonin-rich fruits. In addition, our transgenic studies indicated that tryptamine 5-hydroxylase might be a key enzyme in serotonin biosynthesis. Curling leaf margins were observed in TDX lines, which may result from a decrease in tryptophan concentration. The number of days from flowering to the breaker ripening stage decreased in TDX fruit and wild-type fruit treated with serotonin, and the expression of ripening-related genes was promoted in TDX fruit in real-time PCR and RNA-sequencing analyses, indicating the role of serotonin in ripening. Collectively, our results revealed the horticultural importance of TDC in fruit with its biochemical and physiological roles.