This book constitutes the proceedings of the 16th International Workshop on OpenMP, IWOMP 2020, held in Austin, TX, USA, in September 2020. The conference was held virtually due to the COVID-19 pandemic. The 21 full papers presented in this volume were carefully reviewed and selected for inclusion in this book. The papers are organized in topical sections named: performance methodologies; applications; OpenMP extensions; performance studies; tools; NUMA; compilation techniques; heterogeneous computing; and memory. The chapters 'A Case Study on Addressing Complex Load Imbalance in OpenMP' and 'A Study of Memory Anomalies in OpenMP Applications' are available open access under a Creative Commons Attribution 4.0 License via link.springer.com.