Abstract Objective Exposure to particulate matter less than 2.5μm in diameter (PM2.5) is associated with asthma and respiratory symptoms, but little is known about the influence of PM2.5 on allergic conjunctivitis. The purpose of this study was to examine the association of PM2.5 with outpatient attendance for allergic conjunctivitis. Methods We conducted a time-series analysis of the association between outpatient attendance for allergic conjunctivitis and PM2.5 levels from May to July (non-pollen season) and from August to October (the autumnal pollen season) in 2012. Air pollution data (including the levels of PM2.5, oxidants, nitric oxide, nitrogen dioxide, nitrogen oxide, carbon monoxide, methane, non-methane hydrocarbons, and total hydrocarbons) and data on the daily weather conditions (temperature, wind speed, and humidity) were collected at a centrally located monitoring station in Tokyo. We calculated weekly average values for the daily number of outpatient visits, as well as the air pollution and weather data, and used the weekly average values for analysis. Results There was a significant association between the number of outpatient visits for allergic conjunctivitis and the PM2.5 level (r=0.62, p=0.0177) from May to July, while no correlation was found between the number of outpatient visits and any variable assessed from August to November. Multivariate analysis also showed that PM2.5 was a significant predictor of the number of outpatient visits from May to July (odds ratio=9.05, p=0.0463), while there were no significant predictors of the number of outpatient visits from August to October. From May to July, PM2.5 showed a negative correlation with humidity (r=−0.53, p=0.0499). Conclusions These findings suggest a possible role of PM2.5 in the development of allergic conjunctivitis during the non-pollen season. This association between PM2.5 and allergic conjunctivitis may have broad public health implications in relation to allergic diseases.