As a fundamental concern of human beings, mortality salience impacts various human social behaviors including intergroup interactions; however, the underlying neural signature remains obscure. Here, we examined the neural signatures underlying the impact of mortality reminders on in-group bias in costly punishment combining a second-party punishment task with multivariate pattern analysis of fMRI data. After mortality salience (MS) priming or general negative affect priming, participants received offers from racial in-group and out-group proposers and decided how to punish proposers by reducing their payoffs. We revealed that MS priming attenuated in-group bias and dampened the discriminated activation patterns pertaining to group identities in regions previously implicated in costly punishment, including dorsomedial prefrontal cortex, temporo-parietal junction, anterior cingulate cortex, and dorsolateral prefrontal cortex. The group identity represented in multivariate patterns of activity of these regions predicted in-group bias for the control condition, i.e., the stronger discriminative representations of group identities in these regions; the larger was the in-group bias. Furthermore, the in-group bias was reliably decoded by distributed activation patterns in the punishment-related networks but only in the control condition and not in the MS condition. These findings elucidate the neural underpinnings of the effects of mortality reminders on intergroup interaction. Hum Brain Mapp 38: 1281-1298, 2017. (C) 2016 Wiley Periodicals, Inc.