BACKGROUND: Excessive endoplasmic reticulum (ER) stress in intestinal epithelial cells can lead to damage to the intestinal mucosal barrier, activate the signal transducer and activator of transcription 3 (STAT3)/nuclear factor kappa B (NF-κB) signaling pathway, and exacerbate the inflammatory response, thus participating in the pathogenesis of ulcerative colitis (UC). Mesalazine is a commonly used drug in the clinical treatment of UC. However, further studies are needed to determine whether mesalazine regulates the ER stress of intestinal epithelial cells, down-regulates the STAT3/NF-κB pathway to play a role in the treatment of UC. AIM: To study the therapeutic effects of mesalazine on spontaneous colitis in interleukin-10 (IL-10)(-/-) mice. METHODS: The 24-week-old IL-10(-/-) mice with spontaneous colitis were divided into the model group and the 5-amino salicylic acid group. Littermates of wild-type mice of the same age group served as the control. There were eight mice in each group, four males and four females. The severity of symptoms of spontaneous colitis in IL-10(-/-) mice was assessed using disease activity index scores. On day 15, the mice were sacrificed. The colon length was measured, and the histopathological changes and ultrastructure of colonic epithelial cells were detected. The protein expressions of STAT3, p-STAT3, NF-κB, IκB, p-IκB, and glucose-regulated protein 78 were identified using Western blotting. The STAT3 and NF-κB mRNA expressions were identified using real-time polymerase chain reaction. The glucose-regulated protein 78 and C/EBP homologous protein expressions in colon sections were detected using immunofluorescence. RESULTS: Mesalazine reduced the symptoms of spontaneous colitis in IL-10 knockout mice and the histopathological damage of colonic tissues, and alleviated the ER stress in epithelial cells of colitis mice. Western blotting and quantitative real-time polymerase chain reaction results showed that the STAT3/NF-κB pathway in the colon tissue of model mice was activated, suggesting that this pathway was involved in the pathogenesis of UC and might become a potential therapeutic target. Mesalazine could down-regulate the protein expressions of p-STAT3, NF-κB and p-IκB, and down-regulate the mRNA expression of STAT3 and NF-κB. CONCLUSION: Mesalazine may play a protective role in UC by reducing ER stress by regulating the STAT3/NF-κB signaling pathway.