Cognitive control (CC) over emotional distraction is of particular importance for adaptive human behaviour and is associated with activity in the left dorsolateral prefrontal cortex (dlPFC). Deficient CC, e.g., presenting as negativity bias, has been suggested to underlie many of the core symptoms of major depression (MD) and is associated with impairments of dlPFC function. Correspondingly, enhancement of dlPFC activity with anodal transcranial direct current stimulation (tDCS) can ameliorate these impairments in patients with MD. Here, we tested the hypothesis that a reduction of dlPFC activity by cathodal tDCS induces CC deficits, thus triggering a depression-like negativity bias in healthy subjects. Twenty-eight individuals participated in a double-blinded, balanced randomized crossover trial of cathodal (1 mA, 20 min) and sham tDCS applied to the left dlPFC. To assess CC we conducted a delayed response working memory (DWM) task and an arithmetic inhibition task (AIT) with pictures of varying valent content (negative, neutral, positive) during and immediately after stimulation. Cathodal tDCS led to impaired CC specifically over negative material as assessed by reduced response accuracy in the DWM and prolonged response latency in the AIT. Hence, the current study supports the notion that left dlPFC is critically involved in CC over negative material. Together with previously reported beneficial anodal effects, it indicates that the hypoactivation of left dlPFC causes deficits in CC over negative material, which is a possible aetiological mechanism of depression.