Studies have shown that impairments in the ventromedial prefrontal cortex (vmPFC) in forensic patients who also abuse cocaine and alcohol play a crucial role in are found not to be effective. A promising intervention is to modulate the vmPFC by transcranial Direct Current Stimulation (tDCS). This study aimed to investigate high definition (HD)-tDCS as an intervention to increase empathic abilities in order to reduce violent behavior in forensic patients. In addition, using electroencephalography (EEG), we examined the effects of tDCS on the event-related potentials P3 and LPP in reaction to situations that depict victims of aggression (i.e. empathy). Fifty male forensic substance use patients (25 in the active and 25 in the sham condition) were tested in a double-blind placebo-controlled study. The patients received HD-tDCS two times a day for 20 min for five consecutive days. Before and after the intervention, the patients completed self-reports, performed the Point Subtraction Aggression Paradigm (PSAP), and EEG was recorded while patients performed an empathy task. Results showed a significant decrease in aggressive responses on the PSAP and a decrease in self-reported reactive aggression in the active tDCS group. Additionally, we found an increase in LPP amplitude after tDCS intervention in the active tDCS group compared to the sham group. No effects on the P3 were found. Current findings are the first to find effects of tDCS modulating aggression and electrophysiological responses in forensic patients, thereby showing the potential for tDCS as a intervention in forensic mental healthcare.