Start-ups, among other enterprises, play a major role in the development and/or commercialization of new technologies and the development of national economies, given that firms are the innovation locus for an entire society. In Italy, a recent regulatory intervention has focused on start-ups creating a framework where innovative start-ups are defined and regulated. Among innovative start-ups, those with a social vocation are of particular interest, since they are understudied in the literature. Indeed, the aim of this paper is twofold: to analyze the relationship between social innovation and sustainability in the latter businesses, and try to understand how sustainability could be fostered through them. Italian cases of innovative start-ups will be studied through content analysis applied to the Social Impact Assessment Document provided by firms. Results show that the Social Impact Assessment Document provided by innovative start-ups explicitly pays attention to social innovation and sustainability in different ways. However, the document does not show the link between social innovation and sustainability. Nonetheless, going through these documents, the link between social innovation and the three aspects of sustainability (economic, social and environmental) clearly emerge and therefore could be better managed.