This book constitutes the refereed proceedings of the 11th International Conference on Similarity Search and Applications, SISAP 2018, held in Lima, Peru, in October 2018. The 16 full papers presented together with 3 short papers and 1 demonstration paper were carefully reviewed and selected from 31 submissions. The papers deal with issues surrounding the theory, design, analysis, practice, and application of content-based and feature-based similarity search. They are organized in the following topical sections: metric search; visual search; nearest neighbor queries; clustering and outlier detection; graphs and applications; and shared session SISAP and SPIRE.