Generalized anxiety disorder (GAD) is a debilitating condition, characterized by negative interpretations about ambiguous situations. This study tested whether entirely internet-delivered interpretation training [cognitive bias modification (CBM)] versus control promotes positive interpretations and reduces worry and anxiety in individuals with GAD, with or without depression. A two-arm (CBM; control) parallel-group randomized controlled experiment. Assessments were preintervention (T0), postintervention (T1), 1-month (T2) postintervention, and 3-month (T3) postintervention. Participants with GAD (with or without comorbid depression) were randomly allocated to either CBM ( = 115) or control ( = 115). Participants, but not researchers, were blind to allocated condition. Participants completed up to 10 online CBM or control sessions across 1 month. Interpretation bias [coprimary outcomes: scrambled sentence test (SST), recognition test (RT)], and number of negative thought intrusions during a breathing focus task were measured at T0 and T1. Self-reported levels of worry [Penn State Worry Questionnaire-trait (PSWQ trait); Penn State Worry Questionnaire-past week (PSWQ weekly)], anxiety [Generalized Anxiety Disorder scale (GAD-7)], depression [Patient Health Questionnaire (PHQ-9)], rumination [Ruminative Response Scale (RRS)], and repetitive negative thinking [RNT; Repetitive Thinking Questionnaire-trait (RTQ-trait)] were assessed at T0-T3. The per-protocol analyses included = 186 participants (CBM = 94; control = 92). As predicted, we found moderate-to-large training effects on the primary outcome of interpretation bias at T1. Secondary outcomes of negative thought intrusions at T1 and self-reported symptoms at T2 were all significantly lower in the CBM versus control condition. All but one effect (trait RNT) were sustained at T3. In this randomized controlled study, we found that fully online interpretation training ameliorated core features of GAD in individuals with or without comorbid depression up to 3 months posttraining. (PsycInfo Database Record (c) 2021 APA, all rights reserved).