Objectives To assess the effectiveness of a web‐based brief intervention (BI) program to record daily drinking among people with problem drinking in workplace settings. Methods A two‐armed, parallel‐group, randomized controlled trial were conducted at six workplaces in Japan. After obtaining written consent to participate in the study, workers with an Alcohol Use Disorders Identification Test (AUDIT) score of 8 or higher were randomly assigned into two groups. The participants allocated to the intervention group recorded their daily alcohol consumption for 4 weeks using the program, while those allocated to the control group received no intervention. Outcome measures included the amount of alcohol consumption in past 7 days using the Timeline Follow‐Back method in the program at baseline, 8th week, and 12th week and written AUDIT score at baseline and 12th week. Results Hundred participants were assigned to either the intervention group ( n = 50) or control group ( n = 50). The results of two‐way repeated measures ANOVA showed a statistically significant interaction between the group and the week factors in the two primary outcomes (number of alcohol‐free days, total drinks) and secondary outcomes (AUDIT score) ( p = .04, .02, and .03, respectively). The between‐group effect sizes (Hedges' g ; 95% CI) of the outcomes at 12th week were 0.53; 0.13–0.93 (total drinks), 0.44; 0.04–0.84 (AUDIT score), 0.43; 0.03–0.83 (number of alcohol‐free days). Conclusions The web‐based BI program for problem drinking was considered to be effective in reducing alcohol consumption and the AUDIT score in workplace settings.