Background: Prolonged smartphone use imposes sustained biomechanical loading on the cervical and thoracic spine, yet existing literature has predominantly focused on pain syndromes and postural alterations without adequately examining the underlying sensorimotor control deficits. Sensorimotor noise-the inherent variability in afferent proprioceptive signaling-may accumulate during prolonged cervical flexion, leading to proprioceptive drift and impaired reposition accuracy. Objective: This experimental study examined whether habitual smartphone posture induces sensorimotor noise and proprioceptive drift in cervicothoracic joint position sense, and whether a structured sensorimotor retraining protocol restores reposition precision. Methods: Forty asymptomatic young adults (aged 20-28 years) with more than five hours of daily smartphone use completed multi-angle cervical and thoracic reposition tests. Primary outcomes included absolute reposition error, variable error, constant error, proprioceptive drift index, and sensorimotor noise coefficient. Participants completed a six-week sensorimotor retraining programme (three sessions weekly) incorporating variable-target reposition exercises, closed-chain cervical stabilisation, and cervico-thoracic kinetic chain training. Results: All outcome measures improved significantly (p < 0.001). Cervical reposition error decreased from 6.8° (SD = 1.9) to 2.9° (SD = 0.8) with Cohen's d = 2.68. Thoracic reposition error reduced from 5.4° (SD = 1.6) to 2.3° (SD = 0.7) (d = 2.42). Variable error decreased from 4.2° to 1.6° (d = 2.98). Proprioceptive drift index reduced from 0.68 to 0.28 (d = 3.61), and sensorimotor noise coefficient decreased from 0.82 to 0.34 (d = 3.30). Conclusion: Habitual smartphone posture significantly increases sensorimotor noise and proprioceptive drift in the cervicothoracic spine. Structured sensorimotor retraining effectively restored multi-segment proprioceptive precision, offering a novel rehabilitation framework that addresses the neurophysiological mechanisms underlying smartphoneinduced sensorimotor dysfunction.