Recent trends in strategic management and the strategy-as-practice stream of research have led to a proliferation of studies on open strategy. However, there is a general lack of research focused on valid and reliable measures of open strategy. In this paper, we developed and validated the open strategy scale to measure open strategy constructs derived from two dimensions—transparency and inclusion. We used the mixed methods composed in the multi-phase model of scale development. As a result, we have proposed a multi-item scale to measure the strategy openness. Our results demonstrate the validity and reliability of the scale proposed. The main implication of this research is that the scale may serve as both—an integrated tool for assessment of the overall level of open strategy development and an instrument for more detailed analysis of constructs to reveal the room for improvement or investigate the effect brought by managerial decisions. [ABSTRACT FROM AUTHOR]