The Computational 2D Materials Database (C2DB) is introduced, which organizes a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional (2D) materials. The material properties are systematically calculated using density functional theory (DFT) and many-body perturbation theory (G0W0 and Bethe-Salpeter Equation for approximately 250 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and accessible online or for download. The paper describes the workflow behind the database, presents an overview of the properties and materials available, and explores trends and correlations in the data. Additionally, it identifies a large number of new potentially synthesisable 2D materials with interesting properties for applications in spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and serves as an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.The Computational 2D Materials Database (C2DB) is introduced, which organizes a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional (2D) materials. The material properties are systematically calculated using density functional theory (DFT) and many-body perturbation theory (G0W0 and Bethe-Salpeter Equation for approximately 250 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and accessible online or for download. The paper describes the workflow behind the database, presents an overview of the properties and materials available, and explores trends and correlations in the data. Additionally, it identifies a large number of new potentially synthesisable 2D materials with interesting properties for applications in spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and serves as an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.