Background: Cathepsin S (Cat S) is overexpressed in human atherosclerotic and aneurysmal tissues and may contributes to degradation of extracellular matrix, especially elastin, in inflammatory diseases. We aimed to define the role of Cat S in cardiac inflammation and fibrosis induced by angiotensin II (Ang II) in mice. Methods and Results: Cat S-knockout (Cat S-/-) and littermate wild-type (WT) C57BL/6J mice were infused continuously with Ang II (750 ng/kg/min) or saline for 7 days. Cat S-/- mice showed severe cardiac fibrosis, including elevated expression of collagen I and alpha-smooth muscle actin (alpha-SMA), as compared with WT mice. Moreover, macrophage infiltration and expression of inflammatory cytokines (tumor necrosis factor alpha, transforming growth factor beta and interleukin 1 beta) were significantly greater in Cat S-/- than WT hearts. These Ang II-induced effects in Cat S-/- mouse hearts was associated with abnormal accumulation of autophagosomes and reduced clearance of damaged mitochondria, which led to increased levels of reactive oxygen species (ROS) and activation of nuclear factor-kappa B (NF-kappa B) in macrophages. Conclusion: Cat S in lysosomes is essential for mitophagy processing in macrophages, deficiency in Cat S can increase damaged mitochondria and elevate ROS levels and NF-kappa B activity in hypertensive mice, so it regulates cardiac inflammation and fibrosis.