Supporting teachers professionally, both in their teaching skills and the inclusion of technology, is one of the few direct interventions to help them grow. Currently, few instruments assessing these perceptions exist, and evidence on their validity is sparse. Hence, this paper presents the development and validation of an instrument assessing teachers' perceived institutional support for online and blended learning professional development. A sample of 317 adult education teachers completed the eight-item instrument. Overall, the reliability, exploratory and confirmatory factor analyses suggest that the presented scale can be used to determine how teachers perceive institutional support for online and blended learning professional development and provides evidence supporting a validity argument. Recommendations for the academic and practical use of the ISOBL questionnaire are made.