A growing number of business process management systems is under development both in academia and in practice. These systems typically are based on modern system engineering principles, such as service-oriented architecture. At the same time, the advent of big data analytics has changed the scope of these systems, including functionality such as data mining. However, existing reference architectures for business process management systems date back 20 years and, consequently, are not up-to-date with these modern developments. To fill the gap, this article proposes an up-to-date reference architecture, called BPMS-RA, for modern business process management systems. BPMS-RA is based on analysis of recent literature and of existing commercial implementations. This reference architecture aims to provide a guideline template for the development of modern-day business process management systems by specifying functions and interfaces that need to be provided by these systems as well as a set of quality criteria that they need to meet.