Social interactions are theorized to inform relation-inferred self-efficacy (RISE), which, in turn, may influence self-efficacy and behavior. This study investigated the effects of peer encouragement on RISE, task self-efficacy, and physical performance. Children (
N= 84) were assigned to dyads and randomized to provide peer encouragement to one another or not (control group). Participants completed two endurance handgrip trials, separated by a cognitively demanding task intended to induce mental fatigue and increase the salience of the peer encouragement manipulation. Participants in the experimental group exchanged words of encouragement prior to the second endurance trial, whereas those in the control group did not. The peer encouragement group reported higher RISE and showed increased performance across trials compared with controls. Providing peer encouragement prior to a challenging physical task was associated with more positive RISE perceptions and improved physical performance.