This volume contains extended versions of nine papers presented at the 5th International Workshop on Semantics in Data and Knowledge Bases, which was co-located with ICALP 2011 and organised in Zürich in July 2011. These papers were carefully revised and extended after the workshop and subjected to a thorough reviewing process. The SDKB workshop series tries to bring together researchers in the areas of data and knowledge bases who work on aspects of semantics. In particular, the workshop presents original contributions demonstrating the use of logic, discrete mathematics, combinatorics, domain theory and other mathematical theories of semantics for database and knowledge bases, computational linguistics and semiotics, and information and knowledge-based systems.