GMTSAR is an open source (GNU General Public License) InSAR processing system designed for users familiar with Generic Mapping Tools (GMT). The code is written in C and will compile on any computer where GMT and NETCDF are installed. The system has three main components: a preprocessor for each satellite data type (ERS-1/2, Envisat, ALOS-1, TerraSAR-X, COSMOS-SkyMed, Radarsat-2, Sentinel-1A/B, and ALOS-2) to convert the native format and orbital information into a generic format; an InSAR processor to focus and align stacks of images, map topography into phase, and form the complex interferogram; a postprocessor, mostly based on GMT, to filter the interferogram and construct interferometric products of phase, coherence, phase gradient, and line-of sight displacement in both radar and geographic coordinates; GMT is used to display all the products as pdf files and KML images for Google Earth. A set of shell scripts has been developed for standard 2-pass processing as well as geometric image alignment for stacking and time series. Users are welcome to contribute to this effort.