Being a grand challenge of global scale, the COVID-19 pandemic requires collective and collaborative efforts from a variety of actors to enable the expected scientific advancement and technological progress. To achieve such an open innovation approach, several initiatives have been launched in order to leverage potential distributed knowledge sources that go beyond those available to any single organization. A particular tool that has gained some momentum during COVID-19 times is hackathons, which have been used to unleash the innovation potential of individuals who voluntarily came together, for a relatively short period of time, with the aim to solve specific problems. In this paper, we describe and analyze the case of the hackathon EUvsVirus, led by the European Innovation Council. EUvs Virus was a 3-day online hackathon to connect civil society, innovators, partners, and investors across Europe and beyond in order to develop innovative solutions to coronavirus-related challenges. We have identified four dimensions to explore hackathons as a crowdsourcing tool for practicing effective open innovation in the face of COVID-19: broad scope, participatory architecture, online setting, and community creation. We discuss how these four elements can play a strategic role in the face of grand challenges, which require, as in the case of the COVID-19 pandemic, both urgent action and long-term thinking. Our case analysis also suggests the need to look beyond the ‘usual suspects’, through knowledge recombination with atypical resources (e.g., retired experts, graduate students, and the general public). On this basis, we call for a broader perspective on open innovation, to be extended beyond openness across organizational boundaries, and to explore the role of openness at societal level.