This LNCS conference volume constitutes the proceedings of the 20th International Symposium, ARC 2024, in Aveiro, Portugal, in March 2024. The 16 full papers together with 5 papers from the technical program included in this volume were carefully reviewed and selected from 24 submissions. The conference focuses on the application and development of reconfigurable computing techniques, fault-tolerance, data, and graph processing acceleration to computer security.