This proceedings volume contains selected and expanded contributions presented at the 6th International Symposium of Space Optical Instruments and Applications, held in Delft, the Netherlands on Sep 24th–25th, 2019. The meeting was organized by the Sino-Holland Space Optical Instruments Joint Laboratory and supported by TU Delft. The symposium focused on key innovations of space-based optical instruments and applications, and the newest developments in theory, technology and applications in optics, in both China and Europe. It thus provided a platform for exchanges on the latest research and current and planned optical missions. The major topics covered in these conference proceedings are: space optical remote sensing system design; advanced optical system design and manufacturing; remote sensor calibration and measurement; remote sensing data processing and information retrieval; and remote sensing data applications.