Humans show a pervasive bias for processing self- over other-related information, including in working memory (WM), where people prioritize the maintenance of self- (over other-) associated cues. To elucidate the neural mechanisms underlying this self-bias, we paired a self- vs. other-associated spatial WM task with functional magnetic resonance imaging (fMRI) and transcranial direct current stimulation (tDCS). Maintaining self- (over other-) associated cues resulted in enhanced delay-period activity in classic WM regions (frontoparietal cortex), and in superior multivoxel pattern decoding of the cue locations from visual cortex. Moreover, ventromedial prefrontal cortex (VMPFC) displayed enhanced functional connectivity with WM regions during maintenance of self-associated cues, which predicted individuals’ behavioral self-prioritization effects. In a follow-up tDCS experiment, we targeted VMPFC with either excitatory (anodal), inhibitory (cathodal), or sham tDCS. Cathodal tDCS eliminated the self-prioritization effect. These findings provide strong converging evidence for a causal role of VMPFC in driving self-prioritization effects in WM.