Interactions with animals represent a promising way to reduce the burden of childhood mental illness on a large scale. However, the specific effects of child-animal interactions are not yet well-established. This study provides a carefully controlled demonstration that unstructured interactions with dogs can improve clinically relevant symptoms in children. Seventy-eight children (55.1% female, 44.9% male) ages 10 to 13 (M = 12.01, SD = 1.13) completed the Trier Social Stress Test for Children, followed by (a) interaction with a dog, (b) a tactile-stimulation control condition, or (c) a waiting control condition. The Positive and Negative Affect Schedule for Children, Short Form and the State/Trait Anxiety Inventory for Children were completed at baseline and posttest, and salivary cortisol was assessed at 5 time points. Adjusting for baseline scores, participants in the experimental condition showed higher scores on the Positive Affect scale than participants in both control conditions and lower scores on the State/Trait Anxiety Inventory for Children than participants in the waiting control condition at posttest. Negative affect was not assessed reliably, and we detected no effect of the interactions on salivary cortisol, as measured by area under the curve with respect to ground. Brief, unstructured interactions with dogs boosted children's positive emotions and reduced anxiety. Additional research is needed to further clarify which features of the interactions produce these benefits and the extent to which interactions with animals offer benefits that exceed the effects of other common coping strategies, activities, and interventions.