Transcranial Direct Current Stimulation (tDCS) enhances performance on working memory tasks. However, such effects may be dependent on modulation of specific aspects of working memory. We therefore tested the hypothesis that tDCS improves selective attention in the context of a Sternberg task. Subjects had to maintain a memory set while responding to distracter stimuli. Probes consisted of one item from the memory set, and one item that could have been presented as a distracter. TDCS was found to improve reaction time significantly only when the incorrect choice had been a distracter stimulus. The results thus support the notion that tDCS effects on working memory might be mediated by a specific effect on selective attention.