Open banking policies have been introduced by many governments to foster innovation in financial services, yet evidence on their effects on competition remains limited. I construct a novel dataset of fintech app source code and exploit cross-country variation in open banking availability within the same app to identify the impact of open banking on fintechs. I find that open banking significantly boosts fintech app usage and performance, with the strongest gains for data-intensive apps, fintech startups, and during the pandemic when access to traditional banking was limited. Textual analysis of app descriptions shows that incumbent banks most exposed to open banking suffer declines in loan issuance and income but respond strategically by raising fee intensity and improving earning efficiency. The findings are consistent with theories that open banking reduces barriers to entry by weakening banks' monopoly power over consumer data.