This three-volume set, LNAI 13031, LNAI 13032, and LNAI 13033 constitutes  the thoroughly refereed proceedings of the 18th Pacific Rim Conference on Artificial Intelligence, PRICAI 2021, held in Hanoi, Vietnam, in November 2021.The 93 full papers and 28 short papers presented in these volumes were carefully reviewed and selected from 382 submissions. PRICAI covers a wide range of topics in the areas of social and economic importance for countries in the Pacific Rim: artificial intelligence, machine learning, natural language processing, knowledge representation and reasoning, planning and scheduling, computer vision, distributed artificial intelligence, search methodologies, etc. Part I includes the following topical headings: AI Foundations / Decision Theory, Applications of AI, Data Mining and Knowledge Discovery, Evolutionary Computation / Optimisation, Knowledge Representation and Reasoning.