Human beings naturally prefer and support ingroup members more than outgroup members, but to what extent do we "morally value" equal treatment to ingroups and outgroups? Across four preregistered studies, we examined the development of "group-transcendent fairness," that is, the moral endorsement of allocating resources equally to ingroup members and outgroup members. We found that when allocating common resources to ingroup and outgroup members, American adults (N = 549) thought it was morally right to allocate equally instead of giving more to their family, political, or minimal ingroup members, across high and low stakes (Study 1). This normative sense of group-transcendent fairness develops gradually: 4- to 6-year-olds tended to endorse ingroup favoritism, whereas by age 8 or 9 children endorsed intergroup fairness (Studies 2-3, N = 214). Adults from China (N = 200)--a culture that values ingroup loyalty--also endorsed intergroup fairness as morally right, suggesting this moral value is not specific to western societies where egalitarianism is emphasized (Study 4). In contrast to the normative endorsement of intergroup fairness, participants in all studies did not predict most people to be fair across contexts, suggesting group-transcendent fairness was perceived more as a prescriptive than a descriptive norm (Studies 1-4). Together, our studies reveal the robust presence of group-transcendent fairness, which is valued across group contexts and cultures, develops later than ingroup support, and is prescriptive but not descriptive by nature. The findings help illuminate the nature and development of one group-transcendent moral value that helps promote intergroup relations and societal progress.