Objective:  This single-blind randomized controlled laboratory experiment investigated the impact of resonant breathing on adolescents’ autonomic nervous system (ANS) and affective responses to a standardized acute stressor. Methods:  Adolescents (n=150, 12-13y, 40% boys) were randomized into two conditions: resonant breathing (RB condition) or spontaneous breathing (active control condition), both with visual guidance. In this registered report (https://osf.io/8swdp), mixed-effects models compared conditions on cardiac ANS (heart rate variability RMSSD, pre-ejection period PEP) and affective responses (positive and negative affect) to the Trier Social Stress Test, across reactivity and recovery phases. Additionally, conditions were compared on rumination and reappraisal use. Results:  The determined resonant rate showed a narrow interquartile range (6.2-6.7 breaths/min) and proved difficult to sustain during the 10-minute exercise. The RB intervention significantly reduced breathing rate (Cohen’s d=−4.69) and increased RMSSD during the breathing phase (d=1.68). Compared to the control condition, PEP reactivity to the stressor was attenuated in the RB condition (d=0.32), supporting a protective effect. Unexpectedly, RMSSD stress reactivity was more pronounced in the RB condition (d=−1.37), though this effect diminished in sensitivity analyses, suggesting regression to the mean. Exploratory analyses indicated that among adolescents with lower trait stress-related symptoms, RB reduced RMSSD stress reactivity relative to baseline (d=0.75). No differences between conditions were found in affective responses, emotion regulation, or stress recovery. Conclusions:  The findings only partially support the hypothesis that RB buffers physiological stress reactivity, with no observed effects on affect or recovery. Repeated training may be essential for achieving the resonant rate and potentially enhancing stress-buffering capacity.