Traumatic Brain Injury (TBI) often produces chronic deficits in cognitive function that affect patients' quality of life. To investigate attention-related neural activity differences associated with TBI, we measured whole-head magnetoencephalography (MEG) signals during a cued visuospatial attention task. MEG data from nineteen patients with mild-to-moderate symptomatic TBI and eighteen healthy control participants were analyzed using an adaptive spatial filtering technique (NUTMEG, http://bil.ucsf.edu/nutmeg), and co-registered to individual brain anatomy prior to normalizing images for within-and between-group comparisons. We examine high gamma band activity (63-117 Hz) arising from Incongruently-versus Congruently-cued target stimuli. Using Family-wise Error (FWE) correction for comparisons across voxels, in healthy participants we observe activations in Left Middle Frontal Gyrus (LMFG) from 100 to 175 ms, followed by largely-concurrent Right hemisphere activity from 175 to 300 ms in areas of Inferior Parietal Sulcus (RIPS), Insula, Putamen, and Superior Temporal Cortex and, subsequently, at ~325ms Left Inferior Frontal Cortex for a 50 ms period. In these regions, TBI patients exhibited reduced activity relative to healthy participants and, as a group, showed no alternate compensatory pattern of activity that survived the FWE correction threshold. Linear discriminant analyses revealed that activity levels in just 2 of these 5 regions, LMFG and RIPS, reliably predicted injury status. Furthermore, activity in LMFG showed negative correlation with both the Head Injury Symptom Checklist for post-concussive syndrome (HISC-PCS) and a measure of Cognitive Failures. These results suggest reduced activation due to TBI within a frontotemporal high-gamma oscillatory network subserving spatial attention.