Innovative start-ups are important drivers for economic development. However, they often suffer from several market imperfections and lack the necessary resources to flourish. Incubators are an important policy mechanism for nurturing the creation and growth of successful entrepreneurial ventures. Specifically, we argue that incubators act as an effective tool in filling start-ups’ social capital and in conferring them more legitimacy, so as to ease start-ups’ probability to stipulate alliances with key third parties. In this respect, we also theorize that incubators are heterogeneous and these helping functions may vary with their inherent characteristics. We propose that the supposed ‘bridging effect’ towards start-ups’ alliances could depend on the size of the incubator, its affiliation, and the type of alliance (R&D vs. commercial). The hypotheses are tested through a dataset of 1752 Italian young innovative companies. Results suggest that incubatees are significantly more likely to stipulate alliances with third parties. This bridging role is found not to depend so much on the size of the incubator; conversely, it appears highly contingent on specific matches between the institutional affiliation of the incubator and the type of alliance.