This book constitutes the refereed proceedings of the 27th International Symposium on String Processing and Information Retrieval, SPIRE 2021, held in Lille, France, in October 2021.*The 14 full papers and 4 short papers presented together with 2 invited papers in this volume were carefully reviewed and selected from 30 submissions. They cover topics such as: data structures; algorithms; information retrieval; compression; combinatorics on words; and computational biology. *The symposium was held virtually.