The expansion of High-Speed Rail (HSR) networks has been widely regarded as a key factor in enhancing tourism development, yet its true impact remains a subject of considerable debate. While some studies highlight its role in boosting destination attractiveness, others point to limitations such as displacement effects or the exacerbation of regional inequalities. This paper contributes to this debate by providing new empirical evidence on the impact of HSR on bilateral tourism flows in Spain, a country with one of the world’s most extensive HSR networks. Unlike previous research that often assesses HSR’s effects at an aggregate level, our study adopts a more granular approach. We analyze a panel of monthly bilateral tourism flows between Spanish provinces from 2019 to 2023, using a gravity model to specifically test the effect of direct HSR connections between origin and destination pairs. Our results confirm that the establishment of a direct High-Speed Rail link significantly increases bilateral tourism flows. However, the magnitude of this effect is highly heterogeneous and depends on regional characteristics. We find that rural and non-coastal regions benefit the most from the improved accessibility HSR provides, whereas the effect is considerably weaker or statistically insignificant in metropolitan and coastal areas that are already well-connected. Furthermore, our findings underscore the role of HSR in promoting sustainable tourism by offering a lower-carbon alternative to short-haul flights and car travel. These insights are valuable for policymakers, advocating for a targeted approach to HSR expansion that considers regional disparities, supports balanced territorial development, and advances long-term economic and environmental sustainability goals.