Rates of major depressive disorder (MDD) following traumatic brain injury (TBI) are higher than in the general population. Individuals with depression following traumatic brain injury (TBI-MDD) exhibit working memory (WM) impairments. Electrophysiological evidence has suggested that parieto-occipital upper alpha synchronisation may enhance WM retention by inhibiting irrelevant processes. The current research assessed whether retention period WM parieto-occipital upper alpha activity is disrupted in groups with TBI-only (N=20), MDD (N=17), and TBI-MDD (N=15) compared to healthy controls (N=31). Behavioural data indicated poorer performance in MDD and TBI-MDD. Parietal-occipital upper alpha was reduced in the MDD and TBI-MDD groups, but was unaffected in TBI-only. These results suggest inhibitory deficits may account for WM impairments in MDD and TBI-MDD, and that for individuals with TBI-MDD it may be the depression rather than the TBI that impairs WM.