Exposure-based therapies can be highly effective for patients with post-traumatic stress disorder (PTSD) but up to one third of patients do not respond adequately. Repeated exposure to fear-evoking cues is hypothesized to suppress fear responses through the acquisition and recall of new safety learning in a process termed fear extinction. Transcranial direct current stimulation (tDCS) has gained interest as a non-invasive neuromodulation technique that may suppress fear expression or augment fear extinction. We previously established that in mice, repeated anodal tDCS applied over the prefrontal cortex (PFC) on five consecutive days prior to fear extinction training (offline) can suppress fear expression during extinction training and improve the short-term recall of learned safety. In follow-up experiments we found that the application of anodal tDCS over the PFC during a single extinction training session (online) had no significant effect on the recall of learned safety. However, when anodal tDCS was applied over the PFC immediately before a brief fear retrieval session, we found that tDCS led to increased freezing responses without affecting the long-term fear memory retention. This suggests that the tDCS montage with the stimulation electrode above the PFC and a return electrode fixed to the ventral thorax may have influenced arousal states or fear retrieval, which may have interfered with the fear extinction process. Further investigations should focus on tDCS electrode montages that enable fear extinction training with minimal hindrance for mice.