Addressing the need for conceptualization and operationalization of the social entrepreneurship construct, we propose a behavioral measure of social entrepreneurship orientation (SEO). We build on past qualitative work within the social entrepreneurship literature that contextualizes the behavioral entrepreneurship concept, and draw upon the emergent effectuation theory that captures entrepreneurial behavior in resource-constrained contexts. After surveying 507 U.S.-based social purpose organizations, we offer five dimensions of SEO: innovativeness, proactiveness, risk management, effectual orientation, and social mission orientation. We also find support for SEO's nomological validity, observing a positive influence on social innovation. We discuss limitations and implications of our measure to future research in social-entrepreneurship-led social value creation.