The complete surface temperature (T_c) in urban areas, defined as the mean temperature of the total active surface area, is an important variable in urban micro-climate research, specifically for assessment of the urban surface energy balance. Since most vertically-oriented building facets are not observed by a nadir-viewing remote imaging radiometer, the radiometric surface temperature (T_r) measured at a specific view angle cannot be used with existing heat transfer equations to estimate radiative and convective fluxes in the urban environment. Thus, it is necessary to derive T_c for city neighborhoods. This study develops a simple method to estimate T_c from T_r with the aid of the Temperatures of Urban Facets in 3D (TUF-3D) numerical model, which calculates 3-D sub-facet scale urban surface temperatures for a variety of surface geometries and properties, weather conditions and solar angles. The effects of geometric and meteorological characteristics – e.g., building planar area index (λ_p), wall facet area index (F), solar irradiance – on the difference between T_c and T_r were evaluated using the TUF-3D model. Results showed the effects of geometric and meteorological characteristics on the difference between T_c and T_r differ between daytime and nighttime. The study then sought to predict the relationship between T_r and T_c, using λ_p, F, and solar irradiance for daytime and only using λ_p and F for nighttime. Based on the simulated data from TUF-3D, the resulting relationships achieve a coefficient of determination (r²) of 0.97 and a RMSE of 1.5 K during daytime, with corresponding nighttime values of r² = 0.98 and RMSE = 0.69 K. The relationships between T_r and T_c are evaluated using high resolution airborne thermal images of daytime urban scenes: r² = 0.75 and RMSE = 1.09 K on August 6, 2013 at 12:40 pm; and r² = 0.86 and RMSE = 1.86K on October 24, 2017 at 11:30 am. The new relationships were also applied to estimate T_c from T_r in Hong Kong retrieved from Landsat 5 Thematic Mapper (TM) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). In the present climatic context, the difference between T_c and T_r can reach 10 K during daytime in summer, and 6 K during daytime in winter, with seasonal variation attributable to the variations in shortwave irradiance. The nighttime difference between T_c and T_r can also reach 2 K in both summer and spring seasons.