Promoting prosocial behaviour towards those who are dissimilar from oneself is an urgent contemporary issue. Because children spend much time in same-gender relationships, promoting other-gender prosociality could help them develop more inclusive relationships. Our goals were to better understand the development of school-age children’s intergroup prosocial behavior and the extent to which elementary school-age children consider their own and the recipient’s gender in prosocial behaviour. Participants included 515 3rd, 4th and 5th graders (263, 51.1% boys, M
ageinyears = 9.08, SD = 1.00) surveyed in the fall (T1) and spring (T2). We assessed children’s prosociality using peer nominations. Children became more prosocial toward same-gender peers over time but prosocial behavior toward other-gender peers remained stable. We found that gender mattered: Children showed an ingroup bias in prosociality favouring members of their own-gender group. Having other-gender friendships positively predicted children’s prosocial behaviour towards other-gender peers over time. Children’s felt similarity to other-gender peers was not directly, but indirectly, related to more prosocial behaviour toward other-gender peers. Findings shed light on potential pathways to fostering school-age children’s intergroup prosocial behaviors.