This book constitutes the proceedings of the 21st International Conference on Theory and Practice of Digital Libraries, TPDL 2017, held in Thessaloniki, Greece, in September 2017. The 39 full papers, 11 short papers, and 10 poster papers presented in this volume were carefully reviewed and selected from 106 submissions. In addition the book contains 7 doctoral consortium papers. The contributions are organized in topical sections named: linked data; corpora; data in digital libraries; quality in digital libraries; digital humanities; entities; scholarly communication; sentiment analysis; information behavior; information retrieval.