<p>Open Access (OA) aims to broaden readership, potentially increasing citations and enhancing academic impact. Plan S is a major funder-led initiative to accelerate OA, with its Transformative Journal (TJ) programme providing a key route for hybrid journals. This study estimates the effect of participating in the TJ programme on the academic impact of hybrid OA journals, measured by annual citations per article (CPA). Using a longitudinal dataset of 10,608 hybrid OA journals (2014–2023), we employ a Propensity Score Matched Difference-in-Differences (PSM-DID) design. Journals enrolled in the TJ programme from 2021 constitute the treatment groups, while non-participating hybrid journals form the control group, with the parallel trends assumption validated. The results show that the TJ programme was associated with a statistically significant increase in CPA, with average treatment effects of approximately 5.15% and 2.65% for the two treatment groups. Exploratory mechanism analysis yields findings consistent with the OA ratio serving as a potential mediating pathway, though formal causal mediation analysis would be required to establish this mechanism more definitively. The effects vary by context: no significant difference is found between Europe and North America; the impact is significantly larger for social sciences than for physical sciences; and while smaller publishers benefit in the 2‑year TJ programme, no positive effect is observed for them in the 3‑year TJ programme. These findings suggest that, under the identifying assumptions of the DID framework, the TJ programme might contribute to the academic impact of participating journals. But it might be noted that other concurrent OA initiatives may also influence the effect estimates. This study contributes to a deeper understanding of how policy interventions like Plan S shape scholarly communication, offering evidence-based insights for future policy design.</p>