This book constitutes the refereed proceedings of the 18th International Conference on Similarity Search and Applications, SISAP 2025, held in Reykjavik, Iceland, during October 2025.The 18 full papers and 9 short papers included in the proceedings were carefully reviewed and selected from 58 submissions. The book also contains 2 Doctoral Symposium papers, 3 demonstration papers, and 5 indexing challenge papers. The contributions were organized in the following topical sections: Applications; Foundations; Search and Retrieval; Similarity Measures; Clustering; Indexing; Learning-Based Methods; Doctoral Symposium; Demonstration Papers; and Indexing Challenge.

This book constitutes the refereed proceedings of the 18th International Conference on Similarity Search and Applications, SISAP 2025, held in Reykjavik, Iceland, during October 2025.The 18 full papers and 9 short papers included in the proceedings were carefully reviewed and selected from 58 submissions. The book also contains 2 Doctoral Symposium papers, 3 demonstration papers, and 5 indexing challenge papers. The contributions were organized in the following topical sections: Applications; Foundations; Search and Retrieval; Similarity Measures; Clustering; Indexing; Learning-Based Methods; Doctoral Symposium; Demonstration Papers; and Indexing Challenge.