This paper presents the design of a tool for recurring quantitative self-assessment of IT Service Management (ITSM) process maturity in a financial institution implementing agile software development as a new way of working. This change brought on an increased need to monitor ITSM process performance, and a Design Science Research (DSR) project was launched to create an ITSM maturity assessment tool. Continual improvement of ITSM processes can be measured by performing a process maturity assessment, comparing the organization’s process performance against a best-practice reference set of processes. This paper reports a development project for a quantitative measuring survey-based tool. Due to the increasing use of agile methods, there is an increase in research attention to the coexistence of agile and ITSM maturity assessment tools. The results show that a company-wide ITSM process maturity assessment can be established as a survey-based self-assessment in an agile software development context. The aggregate scores from this self-assessment present a good indicator of the organization’s process performance, especially when complemented by a reference score. A key learning is that the iterative DSR methodology made it possible to create a tool that in good way measure ITSM process maturity in an agile context.