Business model experimentation has been identified as a key driver for business competitiveness but is underexplored in the sustainability and circular economy spheres. What is business model experimentation for the circular economy? This study follows a two-step approach: a literature analysis followed by a qualitative practitioner study. Based on these, circular business model experimentation is defined as an iterative approach to develop and test circular value propositions in a real-life context with customers and stakeholders, starting with a shared goal. It involves rapid learning based on empirical data to provide evidence on the viability of circular value propositions. Iterations involve increased complexity of experiments. There is a learning focus on initiating wider transitions, such as transforming consumer behaviours for the circular economy. We visualise the emerging research landscape, including research streams from business, transitions, engineering, and design. Practically, we illuminate how practitioners view the concept and current experimentation tools and approaches.