This paper presents the application of a simple procedure for estimating long-term average daily reference evapotranspiration (ETo) in Puerto Rico. With only two parameters, site latitude and surface elevation, it is possible to estimate all other input to the Penman-Monteith method. Minimum and maximum air temperatures are estimated from surface elevation data. Dew point temperature is estimated from the minimum temperature plus or minus a temperature correction factor. Temperature correction factors and average wind speeds are associated with six climatic divisions for Puerto Rico. Solar radiation is estimated from a simple equation for island settings or by the Hargreaves’ radiation equation, based on air temperature differences. Estimated ETo is presented for thirty-four locations within Puerto Rico. Comparisons are made with values of ETo previously made using the Hargreaves-Samani method for the same locations.