This paper investigates whether free access to scientific literature increases the participation of under-represented groups in scientific discourse. To this end, we aggregate and match data tracing access to Sci-Hub, a widely used black open access (OA) repository or shadow library, and publication data from the Web of Science (WoS). We treat the emergence of Sci-Hub as an exogenous event granting relatively unrestricted access to publications, which are otherwise hidden behind a paywall. We analyze changes in the publication count of researchers from developing countries in a given journal as a proxy for general participation in scientific discourse. Our results indicate that in the exemplary field of mathematics, free access to academic knowledge is likely to improve the representation of authors from developing countries in international journals. Assuming the desirability of greater international diversity in science (e.g., to generate more original work, reproduce empirical findings in different settings, or shift the research focus toward topics that are overlooked by researchers from more developed countries), our findings lend evidence to the claim of the OA movement that scientific knowledge should be free and widely distributed.