The International Symposium on Experimental Algorithms (SEA) is an international forum for researchers in the area of the design, analysis, and experimental evaluation and engineering of algorithms, as well as in various aspects of computational optimization and its applications (telecommunications, transport, bioinformatics, cryptography, learning methods, etc.). The symposium aims at attracting papers from both the Computer Science and the Operations Research/Mathematical Programming communities. The main theme of the symposium is the role of experimentation and of algorithm engineering techniques in the design and evaluation of algorithms and data structures. Selected contributions to SEA present significant contributions supported by experimental evaluation, methodological issues in the design and interpretation of experiments, the use of heuristics and meta-heuritics, or application-driven case studies that deepen the understanding of the complexity of a problem. Each submission to SEA 2021 was reviewed by at least three Program Committee members or external reviewers. After a careful peer review and evaluation process, 23 papers were accepted for presentation and for inclusion in the proceedings, according to the reviewers’ recommendations. The scientific program of the symposium also includes presentations by three keynote speakers: Dominik Kempa (Johns Hopkins University, USA), Petra Mutzel (University of Bonn, Germany) and Blair D. Sullivan (University of Utah, USA). The 19th edition of the International Symposium on Experimental Algorithms (SEA 2021) was organized by the I3S laboratory (Université Côte d’Azur, CNRS) with the help of Corinne Julien-Haddad. We thanks Université Côte d’Azur, the research center Inria Sophia Antipolis - Méditerranée, and the city of Nice (Comité Doyen Lépine) for their financial support. We also thank the SEA steering committee for giving us the opportunity to host SEA 2021. We express our gratitude to the EasyChair platform. Thanks are also due to the editors of the ACM Journal of Experimental Algorithmics for their interest in hosting a special issue of the best papers presented at SEA 2021. Finally, we express our gratitude to the members of the Program Committee for their support, collaboration, and excellent work.