Summary Background Mesalazine suppositories are recommended and widely used as the standard therapy in induction and maintenance of remission for proctitis. Aim To evaluate the efficacy of mesalazine suppositories in patients with ulcerative colitis (UC) and rectal inflammation; and in patient groups categorised by the extent of lesions. Methods This study was a phase III multicentre, randomised, double‐blind, placebo‐controlled, parallel‐group study. Mild‐to‐moderate UC patients with rectal inflammation were randomly assigned either a 1 g mesalazine or placebo suppository. The suppository was administered in the rectum once daily for 4 weeks. The primary efficacy end point was the rate of endoscopic remission (mucosal score of 0 or 1) after 4 weeks. Results The endoscopic remission rates after 4 weeks in the mesalazine and placebo suppository groups were 81.5% and 29.7%, respectively, and the superiority of mesalazine to placebo was confirmed (P < 0.0001, chi‐squared test). For proctitis, the endoscopic remission rates after 4 weeks were 83.8% and 36.1% in the mesalazine and placebo suppository groups, respectively, and the corresponding rates for all other types of UC were 78.6% and 21.4%, respectively. The superiority of mesalazine to placebo was confirmed in both subgroups (P < 0.0001, Fisher's exact test). The percentage of patients without bleeding was significantly higher in the mesalazine group than the placebo group from Day 3 of treatment (P = 0.0001, Fisher's exact test). Conclusions The effectiveness of mesalazine suppositories in all types of UC patients with rectal inflammation was confirmed for the first time in a double‐blind, placebo‐controlled, parallel‐group study (JapicCTI‐ 111421).