This book constitutes the thoroughly refereed post-conference proceedings of the 21st International Workshop on Job Scheduling Strategies for Parallel Processing, JSSPP 2017, held in Orlando, FL, USA, in June 2017. The 10 revised full papers presented in this book were carefully reviewed and selected from 20 submissions. The papers cover topics in the fields of design and evaluation of new scheduling approaches; performance evaluation of scheduling approaches; workloads; consideration of additional constraints in scheduling systems; scaling and composition of very large scheduling systems; cloud provider issues; interaction between schedulers on different levels; interaction between applications/workloads; experience reports from production systems or large scale compute campaigns. .