The 5-HTTLPR polymorphism in the serotonin transporter gene SLC6A4 has previously been dubbed a plasticity marker. Within the 5-HTTLPR polymorphism, a SNP (rs25531) in the L-allele in the promoter region, affects the transcription efficacy of the SCL6A4, leading to functionally important differences related to serotonin transporter availability in the synapses. 5-HTTLPR has been implicated in magnitude of negative attentional bias, a causal risk factor for depression, and the modifiability of attentional biases both in positive and negative directions. Hence, this genotype may moderate the outcomes of attention bias modification (ABM) targeted at reducing depressive symptoms. We conducted a registered randomized sham-controlled trial of ABM in a sample of 301 participants with a history of Major Depressive Disorder (MDD) who had residual symptoms. They were randomized and underwent 14 days of two daily session of either ABM or sham at home. Of these, 264 provided genetic samples for determining the functionally important variant of the gene SLC6A4. We investigated if the SNP (rs25531) moderated the effect of ABM on symptoms of depression (HDRS, BDI-II) and anxiety (BAI), and attention bias post-intervention. None of the outcomes were moderated by the allelic variation in the promoter region of 5-HTTLPR. Limitations include low level of depressive symptoms, lack of data on ethnicity, current and prenatal level of stress, and early traumatic experiences. The 5-HTTLPR polymorphism did not moderate the effect of ABM on the symptom scales, nor attentional bias. Combination or interactions with other genes may be required for prescribing personalized interventions.