This book presents selected papers from the 21st International Conference on Intelligent Information Hiding and Multimedia Signal Processing, in conjunction with the 5th SSIM 2025, held on October 15–17, 2025, in Taichung City, Taiwan. It is divided into three volumes and discusses latest research outcomes in the field of information technology (IT) including but not limited to information hiding, multimedia signal processing, big data, data mining, bioinformatics, database, industrial and internet of things, and their applications.