IMPORTANCE Myopia in school-aged children is a public health issue worldwide; consequently, effective interventions to prevent onset and progression are required. OBJECTIVE To investigate whether SMS text messages to parents increase light exposure and time outdoors in school-aged children and provide effectivemyopia control. DESIGN, SETTING, AND PARTICIPANTS This randomized clinical trialwas conducted in China from May 2017 toMay 2018, with participants observed for 3 years. Of 528 965 primary school-aged children from Anyang, 3113 were randomly selected. Of these, 268 grade 2 schoolchildren were selected and randomly assigned to SMS and control groups. Data were analyzed from June to December 2021. INTERVENTIONS Parents of children in the SMS group were sent text messages twice daily for 1 year to take their children outdoors. All children wore portable light meters to record light exposure on 3 randomly selected days (2 weekdays and 1 weekend day) before and after the intervention. MAIN OUTCOMES AND MEASURES The co-primary outcomeswere change in axial length (axial elongation) and change in spherical equivalent refraction (myopic shift) from baseline as measured at the end of the intervention and 3 years later. A secondary outcome was myopia prevalence. RESULTS Of 268 grade 2 schoolchildren, 121 (45.1%) were girls, and the mean (SD) age was 8.4 (0.3) years. Compared with the control group, the SMS intervention group demonstrated greater light exposure and higher time outdoors during weekends, and the intervention had significant effect on axial elongation (coefficient, 0.09; 95% CI, 0.02-0.17; P =.01). Axial elongation was lower in the SMS group than in the control group during the intervention (0.27mm[95% CI, 0.24-0.30] vs 0.31mm[95% CI, 0.29-0.34]; P =.03) and at year 2 (0.39 mm[95% CI, 0.35-0.42] vs 0.46mm[95% CI, 0.42-0.50]; P =.009) and year 3 (0.30mm [95% CI, 0.27-0.33] vs 0.35mm[95% CI, 0.33-0.37]; P =.005) after the intervention. Myopic shift was lower in the SMS group than in the control group at year 2 (-0.69 diopters [D] [95% CI, -0.78 to -0.60] vs -0.82 D [95% CI, -0.91 to -0.73]; P =.04) and year 3 (-0.47 D [95% CI, -0.54 to -0.39] vs -0.60 D [95% CI, -0.67 to -0.53]; P =.01) after the intervention, as wasmyopia prevalence (year 2: 38.3%[51 of 133] vs 51.1% [68 of 133]; year 3: 46.6%[62 of 133] vs 65.4%[87 of 133]). CONCLUSIONS AND RELEVANCE In this randomized clinical trial, SMS text messages to parents resulted in lower axial elongation andmyopia progression in schoolchildren over 3 years, possibly through increased outdoor time and light exposure, showing promise for reducing myopia prevalence.