Objective: Problematic alcohol use is the third leading contributor to the global burden of disease, partly because the majority of problem drinkers are not receiving treatment. Internet-based alcohol interventions attract an otherwise untreated population, but their effectiveness has not yet been established. The current study examined the effectiveness of Internet-based therapy (therapy alcohol online; TAO) and Internet-based self-help (self-help alcohol online; SAO) for problematic alcohol users. Method: Adult problem drinkers (n = 205; 51% female; mean age = 42 years; mean Alcohol Use Disorders Identification Test score = 20) were randomly assigned to TAO, SAO, or an untreated waiting-list control group (WL). Participants in the TAO arm received 7 individual text-based chat-therapy sessions. The TAO and SAO interventions were based on cognitive-behavioral therapy and motivational interviewing techniques. Assessments were given at baseline and 3 and 6 months after randomization. Primary outcome measures were alcohol consumption and treatment response. Secondary outcome measures included measures of quality-of-life. Results: Using generalized estimating equation regression models, intention-to-treat analyses demonstrated significant effects for TAO versus WL (p = 0.002) and for SAO versus WL (p = 0.03) on alcohol consumption at 3 months postrandomization. Differences between TAO and SAO were not significant at 3 months postrandomization (p = 0.11) but were significant at 6 months postrandomization (p = 0.03), with larger effects obtained for TAO. There was a similar pattern of results for treatment response and quality-of-life outcome measures. Conclusions: Results support the effectiveness of cognitive-behavioral therapy/motivational interviewing Internet-based therapy and Internet-based self-help for problematic alcohol users. At 6 months postrandomization, Internet-based therapy led to better results than Internet-based self-help. (Contains 4 tables and 2 figures.)