The spatio-temporal characteristics of the clearness index (K_T) were investigated using daily global solar irradiance measurements (290–2800 nm) for the period of 2000–2014 at 21 sites in Korea, a complex region in East Asia with a distinct monsoon season and heavy aerosol loading year-round. The annual mean K_T value for all sites is 0.46, with values of 0.63 and 0.25 for clear and overcast skies, respectively. The seasonal variations in monthly average K_T show a minimum of 0.37 in July at all sites except for Jeju, where the value was 0.29 in January. The maximum value (K_T = 0.51) is observed in October, followed by a secondary peak (K_T = 0.49) during February–April. The lowest K_T value (K_T = 0.42) was observed at both the Seoul and Jeju sites, and the highest (K_T = 0.48) in the southeastern regions. Increases in average K_T exceeding 4% per decade were observed in the middle and southeastern regions, with the maximum (+8% per decade) at the Daegu site. Decreasing trends (<−4% per decade) were observed in the southwestern regions, with the maximum (−7% per decade) at the Mokpo site. Cloud amount, relative humidity, and aerosol optical depth together explained 57% of the variance in daily mean K_T values. The contributions of these three variables to variations in K_T are 42%, 9% and 6%, respectively. Thus, the variations in K_T in Korea can be primarily attributed to the presence of clouds and water vapor, with relatively weak aerosol effects.