<p>In this randomized, double-blind, controlled trial, we examined whether combining transcranial direct current stimulation (tDCS) with exposure–response prevention (ERP) (tDCS + ERP) enhances obsessive–compulsive disorder (OCD) treatment efficacy compared to ERP alone. Fifty-three patients with OCD with contamination-related symptoms were enrolled and randomly assigned to receive either active or sham anodal tDCS targeting the prefrontal cortex during ERP treatment, with the intervention comprising ten sessions over an eight-week primary endpoint. An electroencephalogram (EEG) was administered at baseline and after tDCS + ERP treatments 1 and 8, and magnetic resonance imaging was conducted at baseline. The primary outcome was the percent reduction in Yale–Brown Obsessive–Compulsive Scale (Y-BOCS) scores. We found a significantly larger percent reduction in Y-BOCS scores in the active group compared with the sham group after tDCS + ERP treatment 4 (active, 25.3 ± 2.2%; sham, 18.0 ± 2.2%, 95% confidence interval (CI), 1.1–13.5%, <i>F</i> = 5.681, Bonferroni-corrected <i>P</i> = 0.021) and treatment 8 (active, 38.1 ± 3.0%; sham, 28.2 ± 3.0%, 95% CI, 1.3–18.4%, <i>F</i> = 5.406, Bonferroni-corrected <i>P</i> = 0.024). The secondary outcome measure was the response rate, defined as a reduction of ≥35% in the Y-BOCS score, which was 16/26 for patients treated with active tDCS and 8/27 for sham tDCS (active versus sham: 61.5% versus 29.6%, <i>χ</i>² = 5.443, <i>P</i> = 0.020). No moderate or severe adverse effects were reported. An exploratory analysis of EEG-based brain state showed a marginally significant correlation (false discovery rate-corrected <i>P</i> = 0.08) between the final treatment outcome and tDCS-induced changes in microstate class A occurrence after the first active tDCS session. Concurrent tDCS could significantly enhance ERP’s therapeutic efficacy for contamination-related OCD. ClinicalTrials.gov registration: <a href="https://clinicaltrials.gov/study/NCT04527302">NCT04527302</a>.</p>