Based on information processing models of anxiety and depression, we investigated the efficacy of multiple sessions of online attentional bias modification training to reduce attentional bias and symptoms of anxiety and depression, and to increase emotional resilience in youth. Unselected adolescents (N = 340, age: 11-18 years) were randomly allocated to eight sessions of a dot-probe, or a visual search-based attentional training, or one of two corresponding placebo control conditions. Cognitive and emotional measures were assessed pre- and post-training; emotional outcome measures also at three, six and twelve months follow-up. Only visual search training enhanced attention for positive information, and this effect was stronger for participants who completed more training sessions. Symptoms of anxiety and depression reduced, whereas emotional resilience improved. However, these effects were not especially pronounced in the active conditions. Thus, this large-scale randomized controlled study provided no support for the efficacy of the current online attentional bias modification training as a preventive intervention to reduce symptoms of anxiety or depression or to increase emotional resilience in unselected adolescents. However, the absence of biased attention related to symptomatology at baseline, and the large drop-out rates at follow-up preclude strong conclusions.