Sexual selection may affect human altruistic behaviour. Evolutionary psychology predicts that human mate preference reflects sexual selection. We investigated sex differences in preference for opposite-sex altruism according to recipient because the reasons for altruistic behaviour differ according to the relationship between actor and recipient. We employed the Self-Report Altruism Scale Distinguished by the Recipient, which was newly developed to evaluate altruism among Japanese undergraduates. We asked participants to evaluate preferences for each item based on the recipient of the altruistic behaviour (family members, friends or acquaintances, and strangers). Preference for opposite-sex altruism differed according to recipient, gender of the participant, and relationship type, and several significant interactions were observed among these factors. We suggest that whereas women use a potential partner's altruism towards strangers as a costly signal of their resource-holding potential when choosing a mate, they consider altruism towards family when they are in a long-term relationship to ensure that resources are not allocated to non-relatives.