Geographic Information Systems (GIS) is a kind of computer software that allows people to work with digital maps in a fast and flexible way. In the past decade, more and more geography teachers have become interested in the possibilities of using GIS in secondary education. However, teaching with GIS is complex, and little is known about how to do so in an optimal way. Therefore, an Educational Design Research study (EDR) was conducted with the aim to explore the characteristics of an optimal design for GIS-supported geographic inquiry projects. In this EDR study, a project was developed via progressive cycles of designing, testing, and evaluating, together with teachers from different schools. This paper summarizes the outcomes of the EDR study, and presents some design principles for GIS-supported inquiry-based geography education. Teachers could use these design principles to design and conduct GIS-supported geographic inquiry projects, and in such a way raise their geography lessons to a higher level. This paper also shows that although GIS provides many opportunities for enhancing inquiry-based geography projects, it also holds many conditions for its use to be optimal. GIS-supported inquiry-based geography education requires more than providing appropriate software, tasks, and coaching to ensure that students do not get stuck. In order to effectively raise students' geographic thinking to a higher level, the project should offer a considerable amount of guidance: it should include several preparatory and evaluative tasks based on a good domain-specific theory for use in educational settings. In addition, teachers should coach students in structuring, correcting, and expanding their geographic thinking via dialogical teaching. (Contains 2 tables and 7 figures.)