Sinapic acid is a member of the phenylpropanoid family and is abundant in cereals, nuts, oil seeds, and berries. It exhibits a wide range of pharmacological properties. In this study, we investigated the hepatoprotective and antifibrotic effects of sinapic acid on dimethylnitrosamine (DMN)-induced chronic liver injury in rats. Sinapic acid remarkably prevented DMN-induced loss of body weight. This was accompanied by a significant increase in levels of serum alanine transaminase, aspartate transaminase, and liver malondialdehyde content. Furthermore, sinapic acid reduced hepatic hydroxyproline content, which correlated with a reduction in the expression of type I collagen mRNA and histological analysis of collagen in liver tissue. Additionally, the expression of hepatic fibrosis-related factors such as α-smooth muscle actin and transforming growth factor-β1 (TGF-β1), were reduced in rats treated with sinapic acid. Sinapic acid exhibited strong scavenging activity. In conclusion, we find that sinapic acid exhibits hepatoprotective and antifibrotic effects against DMN-induced liver injury, most likely due to its antioxidant activities of scavenging radicals, its capacity to suppress TGF-β1 and its ability to attenuate activation of hepatic stellate cells. This suggests that sinapic acid is a potentially useful agent for the protection against liver fibrosis and cirrhosis.