OBJECTIVE: To compare the effectiveness of cervical pessary to vaginal progesterone for the prevention of preterm birth in women with twin pregnancies and short cervix. METHODS: This randomized controlled trial was conducted at My Duc Hospital, Vietnam. Asymptomatic women with twin pregnancies and cervical length less than 38 mm were randomized to Arabin pessary or vaginal progesterone (400 mg once a day) group. The primary outcome was preterm birth at less than 34 weeks of gestation. Secondary outcomes were adverse maternal and perinatal complications. We planned a subgroup analysis according to quartile of cervical length. Analysis was conducted on an intention-to-treat basis. We estimated that the primary outcome would occur in 28.4% of women treated with progesterone. Thus a total sample size of 290 women divided equally into two groups was required to detect a 14% absolute risk difference in the primary outcome between the two groups (power 80%, alpha-error 5%, 10% loss to follow-up). RESULTS: Between March 2016 and June 2017, we randomized 300 women, 150 women in each group. Preterm birth at less than 34 weeks of gestation occurred in 24 (16%) women in the pessary group and 33 (22%) women in the progesterone group (relative risk [RR] 0.73, 95% CI 0.46-1.18). The use of pessary significantly reduced the composite of poor perinatal outcomes (19% vs 27%; RR 0.70, 95% CI 0.43-0.93). In women with cervical length of 28 mm or less (25th percentile), pessary significantly reduced the preterm birth rate at less than 34 weeks of gestation from 46% (16/35) to 21% (10/47) (RR 0.47, 95% CI 0.24-0.90) and significantly improved the composite of poor perinatal outcomes. CONCLUSION: Cervical pessary and 400 mg vaginal progesterone resulted in similar rates of preterm birth at less than 34 weeks of gestation in women with twin pregnancies and cervical length less than 38 mm. CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, NCT02623881.