It is well known that 5-aminosalicylic acid (5-ASA) is the standard first-line treatment for ulcerative colitis (UC). Medication adherence is an important factor in the treatment of UC. We aimed to identify predictors of low adherence to oral 5-ASA in Koreans with UC. Between July 2017 and January 2018, we performed a multicenter, cross-sectional study across 6 University Hospitals in Korea. Medication adherence was assessed using the modified Morisky Medication Adherence Scale (MMAS-8) questionnaire. Our study included 264 patients with UC. Patients were requested to complete the self-reported MMAS-8 questionnaire and a survey assessing sociodemographic data. Adherence was categorized as low (scores<6), medium (scores 6-7), and high (score 8). The mean age of patients was 44±14 years, women comprised 43.6% of the study population and 49.8% of the studied population showed low adherence to oral 5-ASA. Age <40 years, alcohol consumption, and current smoking were significantly associated with low adherence to oral 5-ASA (age <40 years: odds ratio [OR] 1.76, 95% confidence interval [CI] 1.04-2.96, p = .034; alcohol consumption: OR 1.66, 95% CI 1.00-2.74, p = .049; current smoking: OR 4.06, 95% CI 1.08-15.18, p = .038). When data were classified based on gender, we observed that only in men, alcohol consumption and current smoking showed a significant association with low adherence to oral 5-ASA (alcohol consumption: OR 2.14, 95% CI 1.08-4.23, p = .029; current smoking: OR 5.07, 95% CI 1.32-19.41, p = .018). In women, only age <40 years was significantly associated with low adherence to oral 5-ASA (age <40 years: OR 3.71, 95% CI 1.59-8.66, p = .002). Approximately 50% of patients with UC showed low adherence to oral 5-ASA. Predictors of low adherence were age <40 years, alcohol consumption, and current smoking habits. In men, alcohol consumption and current smoking were significant predictors of low adherence, whereas in women only age <40 years was significantly associated with low adherence.