Attention Bias Modification (ABM) targets attention bias (AB) towards threat, which is common in youth with anxiety disorders. Previous clinical trials showed inconsistent results regarding the efficacy of ABM, and few studies have examined the effect of online ABM and its augmented effect with cognitive behavioural therapy (CBT). The aim of the current study was to examine the efficacy of online ABM combined with CBT for children and adolescents with anxiety disorders in a randomised, double-blind, placebo-controlled trial. Children (aged 8–16 years) completed nine online sessions of ABM (n = 28) or online sessions of the Attention Control Condition (ACC; n = 27) over a period of 3 weeks (modified dot-probe task with anxiety disorder-congruent stimuli), followed by CBT. Primary outcomes were clinician-reported anxiety disorder status. Secondary outcomes were patient-reported anxiety and depression symptoms and AB. Results showed a continuous decrease across time in primary and secondary outcomes (ps < .001). However, no differences across time between the ABM and ACC group were found (ps > .50). Baseline AB and age did not moderate treatment effects. Online ABM combined with CBT does not show different efficacy compared with online ACC with CBT for children and adolescents with anxiety disorders.