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) METHODS: The 24-week-old IL-10 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.