In sub-Saharan Africa, traditional supernatural beliefs, including belief in witchcraft, black magic, or fetishism, are widespread. Some have hypothesized that these beliefs help to sustain cooperative behavior in a setting where the state is often absent. Others have documented that, at least at a macro-level, such beliefs are negatively associated with prosocial behavior. We contribute to a better understanding of the causal effects of these traditional supernatural beliefs by using lab-in-the-field experiments in the Democratic Republic of the Congo. Participants complete a range of experimental tasks where one player chooses whether to act in a prosocial manner towards another player. Participants are randomly assigned to another player that has either a strong or weak belief in witchcraft, and this information is known by the players. We find that participants act less prosocially towards randomly-assigned partners who believe more strongly in witchcraft. We also find that antisocial behavior is more socially acceptable and prosocial behavior less socially acceptable when playing with a partner who believes more strongly in witchcraft. Our findings suggest that the negative relationship between witchcraft and prosocial outcomes observed in the data may, in fact, be due to the causal effect of the presence of traditional supernatural beliefs on people’s behavior.