UNLABELLED: Ulcerative colitis (UC) is a chronic inflammatory disease affecting colon. The primary symptoms of UC are diarrhoea with blood and mucus and sometimes associated with pain. AIM: This study aimed to evaluate the effects of diosmin (DIO) on UC. METHODS: UC was induced in rats by rectal administration of acetic acid (AA). The degree of mucosal ulceration and damage was determined by evaluating the disease activity index (DAI) and colon damage index scores. Inflammation, oxidative stress and apoptotic responses were determined by measuring the levels of tumour necrosis factor-α (TNF-α), cyclooxygenase-2 (COX-II), malondialdehyde (MDA) and reduced glutathione (GSH) in addition to myeloperoxidase (MPO) activity and caspase-3 expression. RESULTS: The results of this study indicate that AA caused increase in DAI and colon damage index scores. Also, markers of inflammation (TNF-α, COX-II and MPO) and oxidative stress (MDA and reduced GSH) were significantly elevated. These changes were associated with increases in colon caspase-3 expression. Treatment of rats with two doses of DIO produced a dose-dependent decline in DAI and colon damage index scores. Also, DIO resulted in significant reduction of inflammatory and oxidative stress markers as well as reduced the expression of caspase-3. CONCLUSION: DIO therapy decreased UC development depending on its ability to decreases inflammation, oxidative stress and apoptosis in rat colon.