This paper presents Itzï, a new free software for the simulation of floods. It is integrated with a geographic information system (GIS), which reduces the human time necessary for preparing the entry data and analysing the results of the simulation. Itzï uses a simplified numerical scheme that permits to obtain results faster than with other types of models using more complex equations. In this article, Itzï is tested with three cases that show its suitability to simulate urban floods.