["Clinical Psychology &Psychotherapy, Volume 33, Issue 4, July/August 2026. ", "\nABSTRACT\n\nThis study examined the role of perceived stress in intervention‐related change during a mindfulness‐based intervention (MBI) for emotional distress across two randomized controlled trials. In Study 1, 636 participants (Mage = 31.06, SDage = 9.54; 83.18% female) with high psychological distress were randomized to an MBI group (N = 318) or a Waitlist control group (N = 318), measuring perceived stress, anxiety and depression at baseline and postintervention. Study 2 randomized 607 participants (Mage = 30.13, SDage = 8.15; 82.08% female) with high psychological distress to an MBI group (N = 304) or a Waitlist control group (N = 303), with the same measurements at baseline, Week 3, Week 5 and postintervention. In both studies, linear mixed‐effects models showed greater reductions in all variables in the intervention group than in the Waitlist control group. In Study 1, simple mediation analyses showed changes in perceived stress accounted for part of the intervention‐related reductions in anxiety and depression. In Study 2, parallel process latent growth curve models indicated that changes in perceived stress accounted for substantial proportions of the intervention effects on anxiety and depression. Random intercept cross‐lagged panel models further suggested that perceived stress functioned as a temporally prior process particularly for subsequent depression, whereas the temporal association with anxiety was more bidirectional. Perceived stress may represent an important process underlying the MBI effects on emotional distress, especially in reducing depression. Early reductions in perceived stress may help identify subsequent improvement in anxiety and depression.\nTrial Registration: Study1: Chinese Clinical Trial Registry number: ChiCTR2200057398. Study2: Clinical Trials number: NCT06035003.\n\n"]