Socially assistive robots (SARs) represent a promising resource for efforts to improve children’s mental health and alleviate suffering on a large scale. However, the effects of SARs on clinically relevant domains are not yet well established. Our goal was therefore to provide a proof-of-concept demonstration of the capacity for SARs to alleviate clinically relevant symptoms in children. Such a demonstration might then serve as the foundation for efforts to establish the role of SARs in mental health care. We examined the influence of interactions with an SAR on mood, anxiety, and arousal in a sample of 87 children between the ages of 6 and 9, following exposure to a stressful task. Participants completed the Trier Social Stress Test for Children before interacting with the SAR, interacting with the SAR turned off, or waiting quietly. Participants completed baseline and posttest measures of state anxiety and mood, and salivary cortisol was collected at 5 time points. Children who interacted with the robot showed greater increases in positive mood than children in either of the two control conditions, but did not differ from control participants in terms of negative mood, anxiety, or arousal. SARs may convey benefits for children’s mental health by augmenting positive mood. Future research should examine the processes through which SARs promote positive emotions and the circumstances under which that effect is most potent. Although preliminary, these findings suggest that SARs may provide a highly efficient way to alleviate clinical symptoms in children by increasing positive mood.