This book constitutes the refereed proceedings of the 12th Pacific Asia Workshop on Intelligence and Security Informatics, PAISI 2017, held in Jeju Island, South Korea, in May 2017 in conjunction with PAKDD 2017, the 21st Pacific-Asia Conference on Knowledge Discovery and Data Mining. The 8 revised full papers and one short paper were carefully reviewed and selected from 13 submissions. The papers cover topics such as information access and security, cybersecurity and infrastructure protection, data and text mining, and network based data analytics.