Holding out for a delayed reward in the face of temptation is notoriously difficult, and the ability to do so in childhood predicts diverse indices of life success. Prominent explanations focus on the importance of cognitive control. However, delaying gratification may also require trust in people delivering future rewards as promised. Only limited experimental work has tested this idea, and such studies with children were focused on general reward expectations, so evidence was ambiguous as to whether social trust played a role. The present study provides the first targeted test of a role for social trust in children's willingness to delay gratification. Children observed an adult behave in either a trustworthy or untrustworthy manner toward another adult, then were tested in the classic delay of gratification task by that adult. Children were less likely to wait the full delay period, and waited less time overall, for a reward promised by an untrustworthy adult, relative to children tested by a trustworthy adult. These findings demonstrate that manipulations of social trust influence delaying gratification, and highlight intriguing alternative reasons to test for individual differences in delaying gratification and associated life outcomes. Children's willingness to delay gratification is typically interpreted in terms of cognitive control, but may also require trust in people delivering future rewards as promised. We manipulated preschoolers' impressions of social trust by having them observe an adult either lie or behave honestly with another person, and then measured their willingness to delay gratification for a reward promised by that adult in the classic marshmallow task. Preschoolers waited less time, and were less likely to wait the full 15 min delay period, for rewards that had been promised by an untrustworthy adult relative to a trustworthy adult.