This book constitutes the refereed proceedings of the 11th Russian Supercomputing Days, RuSCDays 2025, held in Moscow, Russia, during September 29–30, 2025.The 37 full papers included in this book were carefully reviewed and selected from 94 submissions. They were organized in topical sections as follows: Supercomputer Simulation; HPC, BigData, AI: Algorithms, Technologies, Evaluation; Distributed Computing; and HPC Education..