Learner metacognition can positively impact learning. However, little is known about how to effectively design gamebased learning environments such that metacognition is promoted in learners. Previous research does not provide sufficiently structured and empirically verified insights for designers and researchers to make informed design decisions. This paper describes the development of a design framework for metacognition in game-based learning. The framework is derived from existing literature and cases, and further elaborated through a formative expert evaluation. For instruction, gameplay, and the integration of both, the resulting framework defines specific design dimensions that indicate the relevant areas in which informed design-decisions are likely to affect learners' metacognition. As such, this framework aids specification of designs, structured comparisons between different designs, and focused research effort in identifying specific design guidelines for metacognition in game-based learning.