The CCIS book constitutes selected papers of the 12th International Conference on Computational Science and Computational Intelligence, CSCI 2025, which took place in Las Vegas, NV, USA, during December 3&5, 2025.A total of 43 papers was carefully reviewed and selected for inclusion in this book from the following tracks: 4 papers from 142 submissions to the Research Track on Parallel & Distributed Computing, CSCI-RTPD; 20 papers from 154 submissions to the Research Track on Computational Science, CSCI-RTCS;  9 papers from 58 submissions to the Research Track on Internet of Things & Internet of Everything, CSCI-RTOT; and 10 papers from 120 submissions to the Research Track on Computational Biology, CSCI-RTCB.The papers were organized in topical sections as follows: Parallel & Distributed Computing and Applications; Computational Science - Theories, Algorithms, and Applications; Internet of Things and Applications; Computational Biology; and Poster Papers.