Tryptophan is an essential amino acid that is metabolized in the intestine by gut bacteria into indole derivatives, including tryptamine. However, little is known about which bacterial tryptophan metabolites directly influence obesity. In this study, we identified tryptamine as a bacterial metabolite that significantly reduced fat mass following the intraperitoneal injection of five bacterial tryptophan end-products in a diet-induced obese mouse model. Interestingly, tryptamine, a serotonin analog, inhibited both lipogenesis and lipolysis in adipose tissue, which was further confirmed in a 3T3-L1 adipocyte cell culture study. Moreover, oral tryptamine supplementation markedly reduced fat mass and improved insulin sensitivity in a long-term, high-fat-diet, pair-feeding model. These studies demonstrate the therapeutic potential of tryptamine, a bacterial tryptophan metabolite, in ameliorating obesity and insulin resistance by directly regulating lipogenesis and lipolysis in white adipose tissue.