Background <p>Outdoor activity is important to prevent myopia progression among schoolchildren, and maintaining the choroidal thickness is important to retard myopia progression. However, no research has reported the effects of short-term intensive outdoor activities on the choroidal thickness. This institution-based prospective observational study aims to assess the impact of outdoor activity for 1 week during a camp program to study ocular parameters including the choroidal thickness.</p> Methods <p>The intensive outdoor activity program included an average of 6.15 ± 2.98&#xa0;h of daily time spent outdoors during the camp on subsequent days for 1 week at a low-altitude mountain camp. Twenty-four children participated in this program.</p> Results <p>The main outcome was the change in the choroidal thickness compared with baseline. The data were measured at the beginning and end of the program over the course of 1 week and the changes analyzed. The mean age of the participants (50% female) was 11.5 ± 0.5 (standard deviation) years, and the mean changes were as follows: the refractive error became more positive, the axial length decreased, and the choroidal thickness (µm) increased, respectively, by 0.21 ± 1.35 diopters (<i>P</i> = 0.742), -0.01 ± 0.02&#xa0;mm (<i>P</i> = 0.241), and 30.7 ± 20.3&#xa0;μm (<i>P</i> &lt; 0.001).</p> Conclusion <p>The results suggested that intensive outdoor activity for only 1 week increased the choroidal thickness in Japanese schoolchildren.</p>