Promoting prosocial behavior toward 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 goal in the present research was to better understand the extent to which elementary-school age children consider their own and the recipient's gender in prosocial behavior. Participants included 515 3rd, 4th, and 5th graders (263, 51.1% boys, M[subscript ageinyears] = 9.08, SD = 1.00) surveyed in the fall (T1) and spring (T2). We assessed children's prosociality using peer nominations. We found that gender mattered: children showed an ingroup bias in prosociality favoring members of their own gender group. Having other-gender friendships positively predicted children's prosocial behavior toward other-gender peers. Children's felt similarity to other-gender peers was not directly, but indirectly, related to their prosocial behavior toward other-gender peers. Findings shed light on potential pathways to fostering school-age children's gender-based prosociality. [This paper was published in "British Journal of Developmental Psychology."]