FREEWAT is an open source and public domain GIS integrated modelling environment for the simulation of water (surface- and ground-water) quantity and quality in surface water and groundwater with an integrated water management and planning module. The plugin itself is based on QGIS and all the other related software. In order to use it, you have to download and install some executable files to run numerical codes (free and public domain software) addressed by FREEWAT (not all of them are necessary), namely: MODFLOW, MODFLOW-OWHM, MF-NWT, MT3DMS, MT3D-USGS, UCODE2014, SEAWAT. FREEWAT (FREE and open source tools for WATer resource management) project received funding mainly by the EU Horizon 2020 research and innovation programme under grant agreement n. 642224 (www.freewat.eu). The project is Coordinated by Rudy Rossetto (Scuola Superiore Sant'Anna). This plugin development has been performed within Work Package 2 (Software development and integration - Lead partner TEA SISTEMI spa, Iacopo Borsi). Main contributors: Iacopo Borsi, Massimiliano Cannata, Matteo Ghetta, Mirko Cardoso, Stephen Mehl, Vincent Picavet, Vincent Mora, Rotman Criollo, Laura Foglia, Eric Vazquez-Sune, Giovanna De Filippis, Violeta Velasco-Mansilla, Manuel JPM Oliveira, Tiago Martins and Rudy Rossetto.