This book constitutes the refereed proceedings of the 9th Pacific Rim International Conference on Artificial Intelligence, PRICAI 2004, held in Auckland, New Zealand in August 2004. The 94 revised full papers and 45 revised poster papers presented together with 3 invited contributions were carefully reviewed and selected from 356 submissions. The papers are organized in topical sections on logic and reasoning, knowledge representation and search, ontologies, planning, constraint satisfaction, machine learning, computational learning, Bayesian networks, evolutionary computing, neural networks, fuzzy logic, data mining, classification and clustering, case-based reasoning, information retrieval, agent technology, robotics, bioinformatics, image processing and computer vision, natural language processing, and speech understanding and interaction.