The GRANDProto300 (GP300) experiment is the first stage of the Giant Radio Array for Neutrino Detection (GRAND) project. It will be deployed between 2020 and 2021 in a radio-quiet area, at 3000 m of altitude at the rim of the Tibetan plateau, over a total surface of 200 km2. The primary goal of GP300 is to demonstrate the viability of the GRAND detection concepts. It will provide a unique test bench to develop and validate new identification and reconstruction techniques for the radio detection of very inclined air-showers, in the perspective of the next stages of GRAND. GP300 also proposes a rich science case, which includes accurate measurements of cosmic-ray and gamma-ray air-showers in the energy range of 30 PeV to 1 EeV, and a wide-field survey of the Epoch of Reionization, and of radio transients such as Giant Radio Pulses and Fast Radio Bursts.