We investigate the extent and impact of common ownership in fintech companies. We document a range of empirical facts about common ownership patterns in fintech markets around the world and discuss their implications for competition law enforcement. Specifically, fintech firms are often not publicly listed companies, and the largest owners in this type of firms are venture capital and other types of private equity investors, as opposed to large asset management firms, which are often the largest owners in publicly listed companies. We show that the extent of common ownership is generally low among privately held fintech start-ups. However, it grows substantially with the fintech firms going public. More dynamic and larger fintech markets are characterized by lower levels of common ownership, while smaller national and product markets show substantially higher ownership overlaps. Accordingly, the estimated effects of common ownership in private fintech firms – measured by the lambdas – are higher in the smaller markets. Yet, these are still relatively very low compared to empirically observed common ownership in public fintech firms in oligopolistic markets. Finally, we comment on how the specific ownership and governance structures of fintech firms may materially influence the magnitude and systemic nature of effects associated with common ownership. On this basis, we advise how antitrust agencies may take into account these factors in their enforcement practice and in the shaping of competition policy.