Background— The serotonergic 5-HT _2B receptor regulates cardiomyocyte development and growth. A putative contribution of this receptor to fibroblast-dependent cardiac function has not been identified. Methods and Results— By mimicking sympathetic stimulation with chronic isoproterenol perfusion in vivo, we found that mice developed a cardiac hypertrophy, which was prevented by exposure to the 5-HT _2B receptor antagonists SB206553 or SB215505 or in 5-HT _2B receptor–knockout mice. The isoproterenol-induced hypertrophy was associated with an increase in the plasma levels of interleukin-1β and tumor necrosis factor-α but not interleukin-6. In contrast, the plasma isoproterenol-induced cytokine increase was not observed in either 5-HT _2B receptor–mutant or wild-type mice perfused with isoproterenol+SB206553. We demonstrated that stimulation of wild-type cardiac fibroblasts by isoproterenol markedly increased the production of the interleukin-6, interleukin-1β, and tumor necrosis factor-α cytokines. Strikingly, we found that this isoproterenol-induced cytokine production was abolished by SB206553 or in 5-HT _2B receptor–knockout fibroblasts. Serotonin also stimulated production of the 3 cytokines in wild-type fibroblasts, which was effectively reduced in 5-HT _2B receptor–knockout fibroblasts. Conclusions— Our results demonstrate for the first time that 5-HT _2B receptors are essential for isoproterenol-induced cardiac hypertrophy, which involves the regulation of interleukin-6, interleukin-1β, and tumor necrosis factor-α cytokine production by cardiac fibroblasts.