AbstractThe purpose of this study was to implement the monthly FAO Penman-Monteith evapotranspiration estimation method in a GIS environment, given the importance of spatial estimation of the evapotranspiration in the rational management of water resources and also the lack of easily applicable methodology in GIS environment. In this context, a Python programming language script was developed for implementing monthly FAO Penman-Monteith evapotranspiration equation, in ESRI's ArcMap environment. This approach has the advantage of being independent of the type of geospatial data and can be applied in point, line and polygon data while at the same time being flexible and time saving without loss of accuracy. The methodology was applied using meteorological data of the 2011 irrigation period (March to September), acquired from the meteorological station located at the campus of Technological Educational Institute of Western Greece at Messolonghi (38o 21’ 58” N; 21o 28’ 41” E), with the results being very satisfactory.