This book constitutes the refereed conference proceedings of the 27th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2020, held in Paderborn, Germany, in June 2020.* The 19 full papers and 2 invited papers presented in this book were carefully reviewed and selected from 41 submissions. They are divided into seven sections, i.e. invited papers; mobile robots; dynamic graphs; network communication; multi-agent systems; communication complexity; and game theory. * The conference was held virtually due to the COVID-19 pandemic.