Background: Studies that examine a link between long-term exposure to ambient particulate matter (PM) and atopic dermatitis (AD) in children are lacking. This longitudinal study aimed to investigate the effect of long-term exposure to PM on AD in infants. Methods: A total of 150 infants diagnosed with AD before age 2 were enrolled and followed until age 3 in Seoul, Korea. Long-term exposures to ambient PM with an aerodynamic diameter ≤10 μm (PM ₁₀) from birth to age 3 were assessed at an individual level. Effects of long-term exposure to PM ₁₀ on AD persistence and sensitization to aeroallergens were evaluated using Cox proportional hazard regression models after adjusting for potential confounders. Results: Out of 150 infants, 54 (36.0%) showed remission of AD at age 3. The risk of AD persistence at age 3 significantly increased with an increase in long-term exposure to PM ₁₀ [Hazard Ratio (HR) = 1.07, 95% Confidence Interval (CI): 1.01−1.12, p = .017 per 1 µg/m ³]. Moderate-to-severe AD at enrollment was more likely to persist at age 3 with increased exposure to PM ₁₀ ( p < .05), whereas the persistence of mild AD was not influenced by PM ₁₀ exposure. Long-term exposure to PM ₁₀ increased the risk of sensitization to pollen (HR = 1.14, 95% CI: 1.02−1.27, p = .021). However, it did not affect sensitization to house dust mites or pet allergens. Conclusions: An early-life long-term exposure to ambient PM ₁₀ in infants with AD decreases remission and increases sensitization to pollen at age 3.