Attention can be directed either voluntarily based on the goals of the individual or involuntarily "captured" by salient stimuli in the immediate environment. Although involuntary capture is a critical means of directing attention, the completion of many common tasks requires our ability to ignore salient, but otherwise irrelevant stimuli while restricting our attention to stimuli that are related to our goals. Here, we report neurophysiological measures of spatial attention in humans that gauge an individual's ability to resist attentional capture from salient but irrelevant information. By measuring the rapid reallocation of spatial attention immediately after the onset of distractors, we observe that the ability to override attentional capture varies substantially across individuals and is strongly predicted by the specific working memory capacity of each person. High-capacity individuals were much more capable of resisting attentional capture than low-capacity individuals, who involuntarily reallocated spatial attention when distractors were present in the display. These results provide evidence that the poor attentional abilities associated with low memory capacity may stem from an inability to override attentional capture in the initial moments after the onset of distracting information.