Deep brain stimulation (DBS) of subcallosal cingulate white matter (SCC) may be an effective approach for treatment-resistant depression (TRD) that otherwise fails to respond to more conventional therapies, but DBS is invasive, costly, and has potential for adverse effects. Therefore, it is important to identify potential biomarkers for predicting antidepressant response before intervention. Resting-state EEG was recorded from 12 TRD patients at pre-treatment baseline, after 4 weeks SCC DBS, and after 24 weeks SCC DBS. Lower frontal theta cordance (FTC) at baseline (and higher FTC after 4 weeks) predicted lower depression severity scores after 24 weeks. Greater FTC increases (baseline–4 weeks) predicted greater decreases in depression severity scores subsequently (4–24 weeks) and over the course of the study (baseline–24 weeks). Predictive relationships were topographically specific to theta cordance for frontal electrodes. Thus, results from this pilot study suggest that baseline FTC and changes early in treatment each have utility as biomarkers for predicting 6-month clinical response to SCC DBS for TRD.