This book constitutes the refereed proceedings of the 9th International Conference on the Theory and Application of Diagrams, Diagrams 2016, held in Philadelphia, PA, USA, in August 2016. The 12 revised full papers and 11 short papers presented together with 5 posters were carefully reviewed and selected from 48 submissions. The papers are organized in the following topical sections: cognitive aspects of diagrams; logic and diagrams; Euler and Venn diagrams; diagrams and education; design principles for diagrams; diagrams layout.