Extant research has found superior performance of firms located in clusters. However, it is unclear whether this is based on mere proximity or other unobserved factors. We extend this literature by developing a framework to examine in what way institutions promote open innovation processes between clustered firms. Specifically, we develop a set of hypotheses to investigate to what extent structural and relational elements in a cluster organization affect the open innovation culture. Our model integrates effects of agglomeration, networks, information asymmetries and trust on open innovation culture. We focus on the underlying organizational norms established in clustered firms in relation to open innovation. Specifically, we measure open innovation culture in terms of not-invented- and not-sold-here syndromes, which is facilitated by the integration of trust and reduced by information asymmetry within the cluster region. We test this framework using novel and unique data from member and non-member firms of a cluster initiative in a German high-tech cluster. Our findings from moderation analysis show that a regulatory body in the cluster significantly influences the emergence of both inbound and outbound open innovation activities by member firms in the cluster initiative through increased effects of trust and information asymmetries. Thereby, our paper contributes to literatures of open innovation, including networks of small and medium sized enterprises (SMEs), and cluster policy.