This book constitutes the proceedings of the 9th International Conference on Mathematical Software, ICMS 2026, held in Waterloo, ON, Canada, during July 20–23, 2026. 

The 52 full papers included in this book were carefully reviewed and selected from 76 submissions. They were organized in topical sections as follows: Automating Mathematical Reasoning: Interactions Between Proof Assistants, Computer Algebra Systems (CAS) and Generative AI; Computational Category Theory; Mathematics in Lean; High-Performance Computer Algebra; Computing Methods in (Dual) Quaternion Matrix/Tensor; Software for Enumerative and Analytic Combinatorics; Biological Applications of Computer Algebra; Design and Implementation of Computer Algebra Systems and Software; Mathematical Knowledge Management; Computational Methods in Combinatorial Algebra and Geometry; Symbolic-Numeric Computation; and General topics.