The Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) is a new concept for a microwave sounder for geostationary applications that has recently been developed at the Jet Propulsion Laboratory (JPL). Sponsored by the NASA Instrument Incubator Program a proof-of-concept prototype has been developed and tested at JPL in collaboration with partners at the NASA Goddard Space Flight Center and the University of Michigan and in close coordination with staff from the NOAA NESDIS Office of System Development as well as NASA to ensure that the GeoSTAR design will meet the needs and measurement requirements of both organizations. The prototype, which is a ground-based fully functional temperature sounder with capabilities similar to the Advanced Microwave Sounding Unit – currently the most advanced microwave sounder operating on polar-orbiting low-earth-orbiting (LEO) satellites, shows excellent performance and serves as the intended proof of concept. This development represents a major breakthrough in remote sensing capabilities, since it finally makes it possible to deploy a microwave sounder as part of the Geostationary Operational Environmental Satellite (GOES) system – a long sought goal. In addition to describing the design concept and discussing test results, we discuss the exciting applications that are possible with a GeoSTAR system. These include the basic sounding functions of temperature, water vapor and liquid water profiling – all possible even under fully cloudy conditions, which is not possible with infrared sounders, and which will soon also be possible under precipitating conditions. In addition, we discuss emerging applications, such as rain and snow rate estimates, ice content and distribution in cloud systems, and convective intensity and atmospheric stability metrics in severe storms and tropical cyclones. These observations will be available as a continuous stream of 2-D “synoptic snapshots” covering the entire visible disc. With these capabilities, GeoSTAR would become the prime hurricane sensor, in addition to providing the basic sounding functions required by the National Weather Service and other operational agencies for regional weather prediction. As has been demonstrated in LEO, microwave sounders are also excellent tools for climate applications, with their superior stability and lack of sampling bias. Additional technology development as well as application studies are now under way, funded by NASA and NOAA, and GeoSTAR will be ready for a space mission in the GOES-R time frame.