A weekend effect in diurnal temperature range (DTR) and its association with aerosol radiative effects are investigated in Korea using long-term surface measurements of the mass concentration of particle matter (PM) (1990–2005) and temperature (1955–2005). Here, we define the weekend effect as the difference between the value of a given variable on Sunday and its average for Tuesday through Thursday. The annual-mean weekend effects in PM concentration are consistently negative (i.e., Sunday < the midweek) in both cities for 1990–2005, with a relatively stronger magnitude in Seoul as compared to other background sites. Although the weekend effect in DTR is vague on an annual mean basis, it is meaningful after being confined to only two seasons (e.g., winter and summer). In winter, the DTR weekend effect largely increased in Seoul, particularly in recent years, while it was not significant in other cities. This regional discrepancy is attributable to the magnitude of the Sunday/midweek difference in the aerosol direct effect that may be stronger in Seoul. Furthermore, the weekly cycles of aerosol optical depth are supportive of this aerosol direct effect in the big cities as compared to the background sites. In summer, the cities exhibited similar positive values of the DTR weekend effect, particularly before 1980, and the values are negatively correlated with the cloud cover weekend effect. This similarity in the DTR weekend effect between the two cities may imply that the aerosol indirect effect plays a more important role in summer over a broad area. Our study reveals that various aerosol mechanisms, geographical settings, and extent of urbanization and population are closely related to the weekend effects, one of anthropogenic climate forcings in urban cities.