AbstractBiomarkers predicting treatment outcome in major depressive disorder could enhance clinical improvement. Here this observational and prospective accuracy study investigates whether an age- and sex-normalized electroencephalography biomarker, based on the individual alpha frequency (iAF), can successfully stratify patients to different interventions such as repetitive transcranial magnetic stimulation (rTMS) and electroconvulsive therapy (ECT). Differential iAF directions were explored for sertraline, as well as rTMS (N = 196) and ECT (N = 41). A blinded out-of-sample validation (EMBARC; N = 240) replicated the previously found association between low iAF and better sertraline response. The subgroup of patients with an iAF around 10 Hz had a higher remission rate following 10 Hz rTMS compared with the group level, while the high-iAF subgroup had highest remission to 1 Hz rTMS and the low-iAF subgroup to ECT. Blinded out-of-sample validations for 1 Hz (N = 39) and ECT (N = 51) corroborated these findings. The present study suggests a clinically actionable electroencephalography biomarker that can successfully stratify between various antidepressant treatments.