Objectives Empirical evidence that mindfulness buffers elite athletes against mental-fatigue decrements remains sparse. This trial examined whether a six-week mindfulness-based intervention (MBI) augments cognitive and sport-specific physical performance in elite handball players in fresh and mentally fatigued states. Design Randomised controlled pretest-posttest training study. Methods Seventy-nine players (56 male, 23 female) completed identical technical and conditioning programmes; 40 were exposed to 2.5 h·wk.-1 of structured mindfulness (mPEAK + Headspace), whereas 39 followed an active control. Pre- and post-testing quantified mind-wandering (SART), response inhibition (Stroop); repeated-sprint ability (RSA) and sport-specific reactive agility (S-RAG) in fresh and mentally fatigued states were assessed. Heart rate, blood lactate, RPE and subjective responses were assessed during training, cognitive and physical performance. Results Relative to controls, the MBI cohort improved SART No-Go accuracy (p = .032) and reduced Stroop reaction time (p = .024). Directional-sprint time (p = .021) and S-RAG completion time (p = .022) were faster, with fewer hand errors (p = .030), especially after mental-fatigue induction. Heart rate, blood-lactate and RPE responses during training and testing were equivalent between groups, indicating that gains were not mediated by physiological adaptation. The MBI group showed lower perceived mental demand and frustration both across the six-week training period (mental demand p = .021; frustration p = .011) and during post-training test batteries (mental demand all ps < 0.021; frustration all ps < 0.019); control ratings were unchanged. Conclusions Integrating 20 min·d-1 of guided mindfulness with standard training enhanced executive function and improved sprint-agility performance in fresh and mentally fatigued handball players, without increasing physiological load.
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