This book constitutes the refereed proceedings of the 11th International Workshop on Arithmetic of Finite Fields, WAIFI 2026, held in Santander, Spain, during June 3&5, 2026. The 21 full papers and 3 Invited papers  included in this book were carefully reviewed and selected from 31 submissions. They were organized in topical sections as follows: Invited talks; Coding Theory and Related Structures; Cryptography and Boolean Functions; Finite Field Arithmetic, Algorithms, and Implementations; and Combinatorics, Curves, and Dynamical Structures.