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Description: Humans must often estimate reward values in the presence of ambiguous information to determine the best course of action. The degree of ambiguity during decision-making has been related to levels of activity in the parietal cortex, however, its specific computations and causal role remain unknown. We tested the hypothesis that the parietal cortex is causally related in the computation of ambiguous probabilities during decision-making, studied via fMRI and concurrent TMS-EEG recordings. We found that the parietal cortex computes the degree of ambiguity assigned to subjective probability estimates. Disruption of these parietal signals selectively increased the assignment of unknown probabilities to choice outcomes, and these effects were accompanied by a modulation of frontal theta oscillations related to prediction error signals emerging from ambiguous choices. These results contribute to evidence supporting a fundamental causal role for the parietal cortex in the computations of ambiguous information during risky decisions and learning in humans.