This book constitutes the proceedings of the 21st European Conference on Advances in Databases and Information Systems, ADBIS 2017, held in Nicosia, Cyprus, in September 2017. The 26 regular papers presented together with one keynote paper and one keynote abstract were carefully selected and reviewed from numerous submissions. The papers are organized in topical sections such as conceptual modeling and human factors; subsequence matching and streaming data; OLAP; graph databases; spatial data management; parallel and distributed data processing; query optimization, recovery, and databases on modern hardware; semantic data processing; and additional database and information systems topics.