It is well-established that cardiac fibrosis contributes to cardiac dysfunction and adverse outcomes. However, the underlying mechanisms remain elusive, warranting further studies to develop new therapeutic strategies. It has been suggested that loureirin B can ameliorate the progression of fibrotic diseases. This study investigated the effects of loureirin B on cardiac fibrosis and explored the underlying mechanisms. Transverse aortic constriction (TAC) was performed to induce cardiac fibrosis in mice. Loureirin B (10 mg/kg/day) or saline was continuously delivered via subcutaneous osmotic mini-pumps. Cardiac fibroblasts (CFs) were treated with angiotensin II (Ang II, 100 nM, 24 h) to simulate fibrosis in vitro. Immunochemistry, echocardiography, and Sircol collagen assays were conducted to evaluate the cardioprotective effects. Quantitative real-time polymerase chain reaction, Western blot, and transfection techniques were performed to elucidate the mechanisms. Results showed that loureirin B prevented cardiac fibrosis and improved cardiac function in mice subjected to TAC. Treatment with loureirin B inhibited the elevation of inflammatory factors (interleukin-18, interleukin-6, and tumor necrosis factor-a), transforming growth factor-81 (TGF-81), and Pin1 induced by TAC. Furthermore, loureirin B treatment inhibited the increased fibroblast activation and collagen synthesis induced by Ang II in CFs. In addition, loureirin B inhibited increased expression of TGF-81 and Pin1 induced by Ang II or TAC. Mechanistically, overexpression of Pin1 induced increased TGF-81 expression and blocked the anti-fibrotic effects in Ang II induced CFs treated with loureirin B. Loureirin B ameliorated cardiac fibrosis and dysfunction both in vitro and in vivo probably through the Pin1/TGF-81 signaling pathway.