Purpose: To investigate the change in the prevalence of reduced visual acuity (VA) in Taiwanese school children after a policy intervention promoting increased time outdoors. Design: Prospective cohort study based on the Taiwan School Student Visual Acuity Screen (TSVAS) by the Ministry of Education in Taiwan. Participants: All school children from grades 1 through 6 were enrolled in the TSVAS from 2001 through 2015. Methods: The TSVAS requires each school in Taiwan to perform measurements of uncorrected VA (UCVA) on all students in grades 1 through 6 every half year using a Tumbling E chart. Reduced VA was defined as UCVA of 20/25 or less. Data from 1.2 to 1.9 million primary school children each year were collected from 2001 through 2015. A policy program named Tian-Tian 120 encouraged schools to take students outdoors for 120 minutes every day for myopia prevention. It was instituted in September 2010. To investigate the impact of the intervention, a segmented regression analysis of interrupted time series was performed. Main Outcome Measures: Prevalence of reduced VA. Results: From 2001 to 2011, the prevalence of reduced VA of school children from grades 1 through 6 increased from 34.8% (95% confidence interval [CI], 34.7%–34.9%) to 50.0% (95% CI, 49.9%–50.1%). After the implementation of the Tian-Tian 120 outdoor program, the prevalence decreased continuously from 49.4% (95% CI, 49.3%–49.5%) in 2012 to 46.1% (95% CI, 46.0%–46.2%) in 2015, reversing the previous long-term trend. For the segmented regression analysis controlling for gender and grade, a significant constant upward trend before the intervention in the mean annual change of prevalence was found (+1.58%; standard error [SE], 0.08; P < 0.001). After the intervention, the trend changed significantly, with a constant decrease by –2.34% annually (SE, 0.23; P < 0.001). Conclusions: Policy intervention to promote increased time outdoors in schools was followed by a reversal of the long-term trend toward increased low VA in school children in Taiwan. Because randomized trials have demonstrated outdoor exposure slowing myopia onset, interventions to promote increased time outdoors may be useful in other areas affected by an epidemic of myopia.