Lithospermic acid (LA), a plant-derived polycyclic phenolic carboxylic acid, is known for its strong anti-inflammatory and antioxidant effects. However, its effects have not yet been studied in ulcerative colitis (UC). This study aimed to assess the protective effects of LA in UC and investigate its potential mechanisms of action. Our findings indicated that LA effectively mitigated oxidative stress in mice with colitis by increasing the production of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-PX), while reducing the levels of malondialdehyde (MDA) and reactive oxygen species (ROS) (p < 0.05 for all). In NCM460 cells, LA inhibited the Lipopolysaccharide (LPS)-induced increase in ROS and preserved the mitochondrial membrane potential. In vitro and in vivo experiments confirmed that LA decreased the production of inflammatory markers (p < 0.05). Additionally, LA upregulated intestinal mucosal proteins, contributing to mucosal barrier repair. Mechanistically, LA activated the nuclear factor erythroid 2-related factor 2 (Nrf2) signalling pathway, increasing the expression of Nrf2, heme oxygenase-1 (HO-1), and NAD(P)H quinone oxidoreductase 1 (NQO1) while inhibiting nuclear factor kappa B (NF-κB) phosphorylation (p < 0.05). Notably, the inhibition of Nrf2 reversed the protective effects of LA against colitis. Molecular docking analyses support a strong interaction between Nrf2 and LA. LA mitigates colitis-related inflammation and oxidative stress mainly by activating the Nrf2 signalling pathway. These findings support the potential development of LA as a novel therapeutic agent for UC.