Changing one’s mind on the basis of new evidence is a hallmark of cognitive flexibility. To revise our confidence in a previous decision, new evidence should be used to update beliefs about choice accuracy, but how this process unfolds in the human brain remains unknown. Here we manipulated whether additional sensory evidence supports or negates a previous motion direction discrimination judgment while recording markers of neural activity in the human brain using fMRI. A signature of post-decision evidence (change in log-odds correct) was selectively observed in the activity of posterior medial frontal cortex (pMFC). In contrast, distinct activity profiles in anterior prefrontal cortex (aPFC) mediated the impact of post-decision evidence on subjective confidence, independently of changes in decision value. Together our findings reveal candidate neural mediators of post-decisional changes of mind in the human brain, and indicate possible targets for ameliorating deficits in cognitive flexibility.