This research investigates whether a short mindfulness exercise can reduce children's psychophysiological stress reactions in the face of a performance task. To answer the question, a randomized controlled trial with 106 elementary school children, aged between 5 and 11 years, was conducted. An intervention group completed a two-minute breathing exercise, whilst a comparison group watched a short video, before both groups performed a stress-evoking Stroop test. The stress levels of both groups were measured via galvanic skin response and compared. It was hypothesized that the comparison group would show a higher stress reaction during the stress-evoking task than the intervention group. Contrary to the hypothesis, results show that the intervention group had a higher psychophysiological stress reaction during the task than the comparison group. However, the stress reaction to the announced difficulty of the task was smaller in the mindfulness group than in the comparison group. Results are discussed based on different theoretical mechanisms of mindfulness. Directions for future research include the use of different techniques and durations of mindfulness interventions, different control group activities and stress-test operationalizations, as well as the distinction between age groups.