RGS5 is a small GTPase activating protein that inhibits Gaq and Gai-mediated signaling downstream of GPCRs. RGS5 is primarily expressed in vascular smooth muscle cells and pericytes, and inhibits AngII and ET-1-mediated signaling to regulate blood pressure and vascular remodeling. Moreover, RGS5 ADX-47273 expression correlates with both cardiac and skin fibrosis, and expression is increased in multiple cancers. We hypothesized that RGS5 controls liver injury via its ability to modulate GPCR-mediated signaling in activated HSCs. In this study, we localize expression of RGS5 to HSCs in the liver, and demonstrate that Rgs5 expression is regulated in both acute and chronic liver injury. Furthermore, mice lacking RGS5 expression develop increased hepatocyte damage and H-89 fibrosis in response to carbon tetrachloride. Rgs5 expression is regulated in cultured HSCs in response to fibrogenic agonists, and ET-1 mediated signaling is potentiated in the absence of Rgs5 expression. Taken together, we demonstrate that RGS5 is a critical regulator of GPCR signaling in HSCs, and controls HSC activation and fibrogenesis in response to liver injury. In this study, we have identified RGS5 as a marker of HSCs and a regulator of GPCR-mediated signaling in the liver. Upregulation of Rgs5 expression correlates with HSC activation during liver injury, and Rgs5 is highly expressed in the fibrotic liver. RGS5 deficient mice develop more severe liver injury following acute CCl4 exposure, and increased fibrosis after chronic CCl4 administration. In vitro, profibrotic and pro-inflammatory mediators regulate Rgs5 expression in HSCs, and RGS5 knockdown in HSCs resulted in increased ERK1/2 signaling in response to ET-1, a possible mechanism for its effects in the liver. Taken together, these data suggest that the regulation of RGS5 in HSCs allows for tunable sensitivity to ET-1 signaling following hepatic injury. Loss of RGS5 disrupts this fine level of control, leading to increased HSC activation, hepatic injury, and fibrosis. The Rgs5LacZ reporter mouse provides a robust and sensitive method to specifically localize RGS5 expression in vivo.