Aims: To test the efficacy of a web-based alcohol intervention with and without guidance. Design: Three parallel groups with primary end-point after 6 weeks. Setting: Open recruitment in the German working population. Participants: Adults (178 males/256 females, mean age 47 years) consuming at least 21/14 weekly standard units of alcohol (SUA) and scoring ≥ 8/6 on the Alcohol Use Disorders Identification Test. Intervention: Five web-based modules including personalized normative feedback, motivational interviewing, goal setting, problem-solving and emotion regulation during 5 weeks. One intervention group received an unguided self-help version (n=146) and the second received additional adherence-focused guidance by eCoaches (n=144). Controls were on a waiting list with full access to usual care (n=144). Measurements: Primary outcome was weekly consumed SUA after 6 weeks. SUA after 6 months was examined as secondary outcome, next to numbers of participants drinking within the low-risk range, and general and work-specific mental health measures. Findings: All groups showed reductions of mean weekly SUA after 6 weeks (unguided: -8.0; guided: -8.5; control: -3.2). There was no significant difference between the unguided and guided intervention (P=0.324). Participants in the combined intervention group reported significantly fewer SUA than controls [B=-4.85, 95% confidence interval (CI)=-7.02 to -2.68, P <0.001]. The intervention groups also showed significant reductions in SUA consumption after 6 months (B=-5.72, 95% CI=-7.71 to -3.73, P <0.001) and improvements regarding general and work-related mental health outcomes after 6 weeks and 6 months. Conclusions: A web-based alcohol intervention, administered with or without personal guidance, significantly reduced mean weekly alcohol consumption and improved mental health and work-related outcomes in the German working population.