ABSTRACT: Importance:Internet and computer game addiction represent a growing mental health concern, acknowledged by the World Health Organization. Objective:To determine whether manualized cognitive behavioral therapy (CBT), using short-term treatment for internet and computer game addiction (STICA), is efficient in individuals experiencing internet and computer game addiction. Design, Setting, and Participants:A multicenter randomized clinical trial was conducted in 4 outpatient clinics in Germany and Austria from January 24, 2012, to June 14, 2017, including follow-ups. Blinded measurements were conducted. A consecutive sample of 143 men was randomized to the treatment group (STICA; n?=?72) or wait-list control (WLC) group (n?=?71). Main inclusion criteria were male sex and internet addiction as the primary diagnosis. The STICA group had an additional 6-month follow-up (n?=?36). Data were analyzed from November 2018 to March 2019. Interventions:The manualized CBT program aimed to recover functional internet use. The program consisted of 15 weekly group and up to 8 two-week individual sessions. Main Outcomes and Measures:The predefined primary outcome was the Assessment of Internet and Computer Game Addiction Self-report (AICA-S). Secondary outcomes were self-reported internet addiction symptoms, time spent online on weekdays, psychosocial functioning, and depression. Results:A total of?143 men (mean [SD] age, 26.2 [7.8] years) were analyzed based on intent-to-treat analyses. Of these participants, 50 of 72 men (69.4%) in the STICA group showed remission vs 17 of 71 men (23.9%) in the WLC group. In logistic regression analysis, remission in the STICA vs WLC group was higher (odds ratio, 10.10; 95% CI, 3.69-27.65), taking into account internet addiction baseline severity, comorbidity, treatment center, and age. Compared with the WLC groups, effect sizes at treatment termination of STICA were d?=?1.19 for AICA-S, d?=?0.88 for time spent online on weekdays, d?=?0.64 for psychosocial functioning, and d?=?0.67 for depression. Fourteen adverse events and 8 serious adverse events occurred. A causal relationship with treatment was considered likely in 2 AEs, one in each group. Conclusions and Relevance:Short-term treatment for internet and computer game addiction is a promising, manualized, short-term CBT for a broad range of internet addictions in multiple treatment centers. Further trials investigating the long-term efficacy of STICA and addressing specific groups and subgroups compared with active control conditions are required. Trial Registration:ClinicalTrials.gov identifier: NCT01434589.