Using the staggered opening of governmental public data platforms in China, we employ the difference-in-difference approach to investigate how public data access affects stock price synchronicity. We find that stock price synchronicity significantly drops after the public data platform is established in a firm’s headquarters city. The underlying mechanism is reducing information acquisition costs rather than increasing market attention or corporate information disclosure quality. Furthermore, the informational role of public data platforms magnifies under higher informed trade risk, poorer corporate governance, or better regional economic and innovation capacity. We highlight the role of public data in facilitating financial market efficiency.