Physical exercise has the potential to influence stress reactivity, but experimental data in children are lacking. The main objective of this randomized cross-over study was to investigate the effect of a moderately-intense exercise bout on children's physiological and psychological reactions to a subsequent acute psychosocial stressor. On two separate laboratory appointments, 114 children (62 female) aged between 10 and 13 years completed the Trier Social Stress Test for Children (TSST-C) and a nonstressful control task, respectively. Fifty-seven participants engaged in an exercise bout prior to the conditions and were matched (based on sex, age, and everyday moderate-to-vigorous intensity physical activity) to a control group, who did not engage in a physically demanding task. Stress reactivity was assessed using saliva cortisol and heart rate (calculated area under the response curve) and self-reported anxiety (post minus pre score). Repeated measures analyses of variance revealed main effects of condition, indicating higher stress reactivity in the TSST-C compared to control for cortisol, heart rate, and anxiety. There were no significant interactions of group and condition. However, a three-way interaction involving chronic stress indicated favorable cortisol reactivity patterns after acute exercise compared to the resting condition, but only for participants with higher levels of chronic stress. While results indicate no generalizable effect of acute exercise on children's physiological and psychological reactivity to acute psychosocial stress, acute exercise seems to transiently counteract chronic stress-induced hyperreactivity of the hypothalamic-pituitary-adrenal axis.