To assess the efficacy of a multiple-sessions Web-based Attentional Bias Modification (ABM) self-help intervention in 434 smokers who made a quit-attempt. Method: Respondents were randomized to receive 6 sessions of ABM- or placebo-training in a period of 2 weeks. Smoking-related cognitions (e.g., self-efficacy and intention to quit) and cognitive biases (i.e., attentional and approach bias) for smoking-cues were assessed before training (pretest). Primary outcome-variable was continued abstinence, 6 months after baseline. Bias reduction at the posttraining assessment was the secondary outcome. A 2 × 2 mixed analysis of variance (ANOVA) and logistic regression analyses were conducted using the whole sample (N = 434) as well as subsamples of light to moderate smokers (<15 cigarettes, N = 115) and heavy smokers (15 or more cigarettes, N = 319). Conservative analyses (coding drop-outs as smokers) as well as complete case analyses were conducted. Results: The ABM training had no significant effect regarding bias reduction and no behavioral effects in the whole sample of smokers. Subsample analyses revealed a significant positive effect on continued abstinence in heavy smokers only (complete case analyses: odds ratio [OR] = 3.15; p = .02; confidence interval [CI] = 1.24–7.99; conservative analyses: OR = 2.49; p = .02; CI = 1.13–5.48). Conclusion: Web-based ABM training is ineffective in fostering cognitive bias reduction and continued smoking abstinence. However, the positive effects in heavy smokers—as indicated by exploratory subsample analyses—warrant further research into the potential of multiple sessions ABM training to foster continued smoking abstinence in heavy smokers who make a quit-attempt.