This study examines the relationship between the diversity of partners that a startup cooperates with and its innovation performance. Open innovation is important for startups as they need to open their boundaries to overcome the liabilities of newness. Based on data from the Spanish Innovation Technology Panel (PITEC) from 2004 to 2013, we find that startups benefit to a greater extent than incumbent firms from cooperation breadth. Moreover, we find that this effect is stronger for high-tech startups in particular. We therefore conclude that the breadth of cooperation partners plays an important role in knowledge exploration and exploitation in startups. This study contributes to the link between open innovation and entrepreneurship, it advances our knowledge on the role of breadth as a mechanism to integrate heterogeneous knowledge and access to complementary assets, and it also sheds light on some of the contingencies in terms of which types of startups will benefit most.