The neural processes underlying the ability to divide attention between multiple sensory modalities remain poorly understood. To investigate the role of the dorsolateral prefrontal cortex (DLPFC) in bimodal divided attention, we completed two repetitive transcranial magnetic stimulation (rTMS) studies. We tested the hypothesis that the DLPFC is necessary in the ability to divide attention across modalities. This hypothesis originated as a result of a previous fMRI study in which the posterior DLPFC was active during a bimodal divided attention condition [Johnson, J. A., & Zatorre, R. J. Neural substrates for dividing and focusing attention between simultaneous auditory and visual events. Neuroimage, 2006]. In the current experiments, two separate groups of subjects underwent 10 min of slow rTMS to temporarily disrupt function of the DLPFC. In both groups, the ability to divide attention between unrelated auditory and visual stimuli decreased following DLPFC disruption compared to control site stimulation. Specifically, the ability to divide attention between modalities was hindered, leading to a pattern of behavior similar to bimodal selective attention (ability to attend to one or the other modality but not both). We discuss possible roles of the posterior DLPFC in bimodal divided attention and conclude that the area may be functioning to support the increased working memory load associated with divided, compared to selective attention.