This book constitutes the proceedings of the 23rd International Conference on Comparative Genomics, RECOMB-CG 2026, held in Thessaloniki, Greece, during May 24–25, 2026.The 19 full papers presented in this book were carefully reviewed and selected from 36 submissions. The papers are divided into the following topical sections: Combinatorics of Genome Rearrangements; Genome Evolution and Reconstruction; Phylogenetics I: Inferring Gene and Species Histories; Phylogenetics II: Compari...