Background and Aims: Estonia has one of the highest alcohol-attributable mortality rates within the European Union. The aim of this study was to estimate the efficacy of an on-line self-help intervention to reduce problem drinking at the population level. Design: On-line open randomized controlled trial with an 8-week intervention and an active control group (intervention n = 303, control n = 286). Assessments took place at baseline and at 6 months follow-up. Setting: On- and offline channels were used for population-based recruitment within a nation-wide prevention campaign in Estonia. Participants: Inclusion criteria were age ≥ 18 years, heavy drinking [Alcohol Use Disorders Identification (AUDIT) test score ≥ 8], literacy in Estonian and at least weekly access to the internet; n = 589 participants were randomized (50% male, 1% other; mean age 37.86 years; 45% with higher level of education). Intervention and comparator: The intervention consisted of 10 modules based on principles of cognitive–behavioral therapy and motivational interviewing. The active control group received access to a website with a self-test including personalized normative feedback and information for standard alcohol treatment. Measurements: The primary outcome was AUDIT scores at 6 months follow-up adjusted for baseline scores. Findings: Intention-to-treat analyses were applied. Missing data were addressed by using baseline observation carried forward (BOCF) and multiple imputation by chained equations (MI); 175 completed follow-up in the intervention group and 209 in the control group. AUDIT score at follow-up was significantly smaller in the intervention [BOCF mean = 13.91, standard deviation (SD) = 7.61, MI mean = 11.03, SD = 6.55] than control group (BOCF mean = 15.30, SD = 7.31; MI mean = 14.30, SD = 7.21), with a group difference of −1.38 [95% confidence interval (CI) = –2.58, –0.18], P = 0.02 for BOCF and −3.26 (95% CI = –2.01, –4.51), P < 0.001 for MI. Conclusions: A randomized controlled trial has found that an on-line self-help intervention with minimal guidance was effective at reducing problem drinking in Estonia.