Repetitive transcranial magnetic stimulation (rTMS) is an established psychiatric procedure for patients suffering from treatment-resistant depression (TRD). Biomarker identification to predict rTMS outcomes may assist the clinician in optimizing treatment selection. In recent years, different electrophysiological markers, in particular electroencephalographic (EEG) markers, were shown to yield discriminative power between responders and non-responders to various TRD treatments. However, so far, predictive markers for the Theta Burst Stimulation (TBS) protocol have remained scarce. The present study, therefore, aimed to identify such markers. Resting state EEGs of 10-15 min were done in a group of 46 TRD patients prior to rTMS TBS treatment (600 stimuli over the left dorsolateral prefrontal cortex). Each patient underwent 19-21 sessions with 4-5 sessions per week. Depression was assessed using the Hamilton Depression Rating Scale and the Beck Depression Inventory II. Our study demonstrated that responders exhibited significantly lower FAA values in a baseline EEG indicating that left frontal alpha dominance was associated with a positive response to TBS-rTMS in TRD patients. FAA was independent of both gender and age. No other biomarker, including alpha peak frequency, or alpha power, showed a significant difference between responders and non-responders. Taken together, FAA observed in EEG readings is emerging as a promising indicator of treatment outcomes in patients with TRD. Given these findings, we suggest considering FAA as a predictive factor when assessing the effectiveness of therapeutic interventions. Further studies replicating these results in larger, diverse populations are needed to confirm FAA as a reliable biomarker of clinical outcome.