OpenSuperQ aims at developing a full-stack quantum computing system of up to 100 qubits and to sustainably make it available at a central site for external users. This system will be applied to tasks of quantum simulation in quantum chemistry which serve as a high-level benchmark, and to problems related to optimization and machine learning. The core of the system will be a processor of superconducting qubits of the planar transmon type, with a square 2D layout and nearest-neighbour connectivity. The processor will be packaged with a control chip and integrated in a custom-made cryogenic system. This quantum computing system will be equipped with integrated control soft- and hardware ready for applications. The computer will be among the leading platforms in the world, and the first of its kind in Europe. A distinguishing and globally unique feature of OpenSuperQ is its open approach ready to serve a large community of users of the system as well as of underlying technologies. The OpenSuperQ consortium works in a collaborative way with broad distribution of tasks and brings together theoretical and experimental teams, a central site including a high-performance computing center, and technology companies. It is supported by an advisory board, a basic science group, and a user board. This broad approach will lead to near-term exploitation, multi-level communication and will develop interfaces and standards made available to the quantum technology community at large.