Public infrastructure projects must comply with the divergent and even conflicting demands of multiple institutional logics causing institutional complexity. Despite the increasing interest in different forms of complexities in projects, we lack empirical illustrations and rigorous theorizing of mechanisms for responding to institutional complexity. This paper demonstrates how public buyers of a tunnel construction project formed a hybrid organization of a multi-party project alliance to respond to institutional complexity. We delineate a process of temporary hybridization through which the competing logics of a bureaucratic state, corporate market, and multiple professions were combined within the temporary project alliance organization. Such temporary hybridization not only focused on selective coupling with external demands but also mitigated internal tensions. Our findings emphasize a blended organizational structure, jointly formed governance and incentive systems, and the facilitation of social interaction to build a temporary yet sustainable hybrid organization capable of combining conflicting institutional logics. •Public infrastructure projects face conflicting institutional demands.•Such institutional complexity can be responded by hybrid forms of organizing.•We conceptualize project alliancing as a hybrid form of organizing.•Based on an empirical case study, we delineate a model of temporary hybridization.