In this study, a model for calculating monthly average hourly diffuse erythemal solar ultraviolet (EUV) radiation under all sky conditions in Thailand was developed. The model is a function of air mass, aerosol optical depth and satellite-derived cloud index, as these parameters are sensitive to diffuse EUV radiation. Diffuse EUV irradiance were measured using UV-biometers equipped with shade ball during April 2011–March 2014 at four stations in Thailand. Data from these stations were used to formulate the model. The model was validated using an independent dataset (April 2014–March 2016) at the same sites. The modeled diffuse EUV irradiance showed good agreement with that from the ground-based measurements, with root mean square difference (RMSD) of 12.7% and mean bias difference (MBD) of −2.8%. The use of satellite data provided a simple and practical method to calculate monthly average hourly diffuse EUV irradiance over Thailand. Spatial monthly distributions are shown as maps.