Key PointsQuestionIs there an association between secondhand smoke (SHS) exposure and childhood myopia?FindingsIn this cross-sectional study that included 12 630 children aged 6 to 8 years, SHS exposure was associated with greater myopic refraction, longer axial length, greater likelihood of developing moderate and high myopia, and earlier myopia onset. The association was greater with higher exposure and younger age.MeaningThese findings suggest that eliminating SHS exposure is important for myopia prevention among children, particularly in families with young children.