The posterior parietal cortex (PPC) has been implicated in perceptual decision-making and categorization, but whether its activity plays a causal role remains controversial. Here we examined the population dynamics of PPC activity during an auditory-guided decision task in mice. We found that silencing of PPC activity impaired several aspects of decision-making. First, categorization of new, but not well-learned, stimuli was impaired. Second, re-categorization of previously experienced stimuli based on newly learned categories was also impaired. Third, the bias on behavioral choices created by preceding trials significantly increased. In vivo two-photon imaging of PPC activity during stimulus categorization revealed differential dynamics in representations of new stimuli and learned categories, consistent with rapid incorporation of new sensory information during categorization. At the circuit level, inactivation of PPC axonal projections to the auditory cortex also significantly reduced categorization performance. Thus, PPC circuits play a causal role in decision-making during stimulus categorization.