BACKGROUND The aim of the present study was to investigate the effect of horizontal whole-body vibration (WBV) training on trunk and lower-extremity muscle tone and activation, balance, and gait in a child with spastic diplegia cerebral palsy. CASE REPORT A 10-year-old male with spastic diplegia cerebral palsy received horizontal WBV training followed by conventional physiotherapy (50 min per day, 12 days per month), but only conventional physiotherapy during followup. Muscle tone was assessed using the Modified Ashworth Scale (MAS) and muscle activation with surface electromyography. Balance was assessed using the Timed Up and Go test (TUG) and Pediatric Balance Scale (PBS), and gait parameters were assessed using the GAITRite system. Assessment was performed at 3 points: pre-intervention, post-intervention, and follow-up. Following the intervention, MAS decreased in both the hip extensor and right ankle plantar flexor. Muscle activation increased post-intervention in the bilateral erector spinae (ES), rectus abdominis (RA), rectus femoris (RF), and right tibialis anterior (TA) during standing, and in the left RA, bilateral RF, gastrocnemius (GCM), and left TA during squatting. At follow-up, activation increased in the right ES, left RA, and RF during standing. At post-intervention and follow-up, improvement was observed in PBS score, gait velocity, right step length, and right stride length, with decreased single-leg support time, and double support and toe deviation angle. CONCLUSIONS Horizontal WBV training can safely and effectively maintain and improve physical performance and can be considered for inclusion in rehabilitation programs.