The aim of this study was to develop and validate a multidimensional open thinking scale (OTS) in order to measure adult learners' open thinking as a key learning outcome of open educational practices (OEP) through a three-phase process of item generation, theoretical analysis, and psychometric analysis. Exploratory and confirmatory factor analyses of data from 610 students in 24 countries revealed a clear structure consisting of six constructs of open thinking: "openness to multiple perspectives," "openness to new learning," "openness to collaboration," "openness to sharing," "openness to change," and "openness to diversity and inclusion." Known-groups validity tests showed that the more frequently a learner is exposed to OEP, the higher the OTS scores obtained. Results suggest that OTS could contribute to OEP and research by defining and elaborating on the concept of open thinking, exploring its relationship to learners' OEP experiences, and measuring and developing students' open thinking.