Young children share fairly and expect others to do the same. Yet little is known about the underlying cognitive mechanisms that support fairness. We investigated whether children’s numerical competencies are linked with their sharing behavior. Preschoolers (aged 2.5–5.5) participated in third-party resource allocation tasks in which they split a set of resources between 2 puppets. Children’s numerical competence was assessed using the Give-N task (Sarnecka & Carey, 2008; Wynn, 1990). Numerical competence—specifically knowledge of the cardinal principle—explained age-related changes in fair sharing. Although many subset-knowers (those without knowledge of the cardinal principle) were still able to share fairly, they invoked turn-taking strategies and did not remember the number of resources they shared. These results suggest that numerical cognition serves as an important mechanism for fair sharing behavior, and that children employ different sharing strategies (division or turn-taking) depending on their numerical competence.