This paper describes a simple method for estimating long-term average monthly climate data for tropical island environments. These data (i.e., minimum and maximum air temperature, dew point temperature, wind speed, and solar radiation) are needed as input to the Penman-Monteith method for calculating reference evapotranspiration. The climate data can be estimated from just two site parameters (latitude and elevation) and specification of a climate division (or zone). In this study, the climate data estimation procedure was applied to Puerto Rico. Comparison of reference evapotranspiration determined by using the proposed method and measured climate data from four locations in Puerto Rico showed good agreement. This method may serve as a means for estimating reference evapotranspiration in other Caribbean Islands.