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.