Welcome to the 9th Swedish Workshop on Data Science (SweDS21) held (virtually) in Växjö, Sweden during December 2–3, 2021. SweDS is a national event with a focus of maintaining and developing Swedish data science research and its applications by fostering the exchange of ideas and promoting collaboration within and across disciplines. This annual workshop brings together researchers and practitioners of data science working in a variety of academic, commercial, industrial, or other sectors. The current and past workshops have included presentations from a variety of domains, e.g., computer science, linguistics, eco- nomics, archaeology, environmental science, education, journalism, medicine, healthcare, biology, sociology, psychology, history, physics, chemistry, geography, forestry, design, and music. SweDS is hosted by Linnaeus University (Växjö, Sweden) this year. Due to the yet ongoing COVID-19 pandemic, travel restrictions, and public health concerns, the workshop is conducted online-only, which has allowed authors both within and outside of Sweden to submit and present their work.