Research Findings: Studies of children's prosocial behavior typically focus on prosocial acts with a specific partner (e.g., a friend, peer, or sibling), and comparisons of prosociality in different contexts are rare. To address this gap, the current study examined predictive links among children's spontaneous sharing (a common and important form of prosocial behavior) with siblings, friends, and unfamiliar peers. At ages 3 and 6 years, 81 children (48 boys) were filmed playing with an older sibling (44 brothers); at age 6 the children were also filmed playing with a best friend (at school) and with 2 unfamiliar peers (in a lab). Older siblings shared more frequently than the target children at the 1st time point, but this contrast was not apparent at the 2nd time point. Average rates of sharing with a sibling did not differ by gender at the 1st time point, but girls shared more frequently than boys at the 2nd time point. When effects of gender composition were controlled, early spontaneous sharing with an older sibling predicted later sharing with unfamiliar peers (but not with a friend). Practice or Policy: Learning to share with a sibling can help children acquire the prosocial skills needed to form positive relationships with their peers.