The two-volume set LNCS 13956 and 13957 constitutes the refereed proceedings of the 23rd International Conference on Computational Science and Its Applications, ICCSA 2023, held at Lesvos Island, Greece, during July 3-6, 2023. The 67 full papers and 13 short papers and 6 PHD showcase papers included in this volume were carefully reviewed and selected from a total of 283 submissions. The contributions are grouped in topics which deal with General Track 1: Computational Methods, Algorithms and Scientific Applications; General Track 2: High Performance Computing and Networks; General Track 3:...