This study aims to evaluate the effectiveness of a web-based intervention to reduce alcohol consumption among hazardous drinkers. A two-group parallel randomized controlled trial was conducted among adults identified as hazardous drinkers according to the Alcohol Use Disorders Identification Test. The intervention delivers personalized normative feedback and some general information about alcohol. Participants can review their motivations and fears regarding reducing their alcohol intake, set individual goals and monitor their progress via a consumption diary and other tools. Within the control group, participants were provided with the same diary but could not access other services from the program. The primary outcome measure was the absolute difference in weekly alcohol intake (WAI) between baseline and 6-week follow-up. Secondary outcome measures included: relative difference in WAI; difference in excessive drinking and significant WAI reduction (decrease of 10% or more in WAI). One thousand one hundred and forty-seven people participated in the trial and 339 subjects completed it. Relative to the control group, participants in the intervention group reported a significantly greater mean absolute reduction in WAI (-3.3 versus -1.2, P = 0.03). Secondary outcomes also presented significant effects. This trial provides preliminary support to the effectiveness of this program in helping hazardous drinkers reduce their drinking, provided it is completely and regularly used.