Understanding the molecular mechanisms that govern hepatic stellate cell (HSC) fate is essential for developing effective antifibrotic therapies. Yes-associated protein (YAP) plays a pivotal role in organ fibrogenesis and HSC activation, but its upstream regulatory signals in liver fibrosis remain largely unknown. Here, we demonstrate the role of p21-activated kinase 4 (PAK4), an emerging therapeutic target, in the activating phosphorylation of YAP and subsequent HSC activation. PAK4 expression was markedly upregulated in activated HSCs from patients and mice with liver fibrosis and correlated with disease severity. Silencing PAK4 suppressed hallmark features of HSC activation, including fibrotic activity, proliferation, migration, and contractility. HSC-specific deletion of Pak4 and oral administration of a selective PAK4 inhibitor attenuated hepatic fibrosis in mice. Mechanistically, PAK4 directly phosphorylated YAP at T428, a previously unrecognized site, leading to YAP stabilization, nuclear accumulation, and transcriptional activation, which in turn promoted HSC activation. Overexpression of a phospho-inactive YAP mutant in HSCs reduced fibrogenesis, validating the functional importance of PAK4-mediated YAP phosphorylation. Notably, T428-phosphorylated YAP levels were substantially elevated in livers from patients with fibrosis. Our findings reveal a new PAK4-YAP signaling axis in HSCs, whereby PAK4-mediated T428 phosphorylation of YAP promotes its activation and drives liver fibrosis. These insights highlight a potential therapeutic strategy targeting HSC activation.