This book constitutes the thoroughly refereed post-conference proceedings of the 15th International Workshop on Coalgebraic Methods in Computer Science, CMCS 2020, colocated with ETAPS 2020, held in Dublin, Ireland, in April 2020. The conference was held as several online events in September and October due to the COVID-19 pandemic. The 9 revised full papers were carefully reviewed and selected from 13 submissions. Also included is one invited talk. The papers cover a wide range of topics in the theory, logics, and applications of coalgebras.