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1. Our knowledge of the functional role of large herbivores is rapidly expanding, and the impact of grazing on species co‐existence and non‐native species expansion has been studied across ecosystems. However, experimental data on large grazer impacts on plant invasion in aquatic ecosystems are lacking. 2. Since its introduction in 2002, the seagrass species Halophila stipulacea has rapidly expanded across the Eastern Caribbean, forming dense meadows in green turtle (Chelonia mydas) foraging areas. We investigate the changes in seagrass species co‐existence and the impacts of leaf grazing by green turtles on non‐native seagrass expansion in Lac Bay (Bonaire, Caribbean Netherlands). 3. Green turtle grazing behavior changed after the introduction of non‐native seagrass to Lac Bay in 2010. Field observations, together with time‐lapse satellite images over the last four decades, showed initiation of new grazing patches (65 ha, an increase of 72%). The sharp border between grazed and ungrazed seagrass patches moved in the direction of shallower areas with native seagrass species that had previously (1970‐2010) been ungrazed. Green turtles deployed with Fastloc‐GPS transmitters confirmed high site fidelity to these newly cropped patches. In addition, cafeteria experiments indicated selective grazing by green turtles on native species. These native seagrass species had significantly higher nutritional values compared to the non‐native species. In parallel, exclosure‐experiments showed that non‐native seagrass expanded more rapidly in grazed canopies compared to ungrazed canopies. Finally, in six years from 2011‐2017, H. stipulacea underwent a significant expansion, invading 20 to 49 fixed monitoring locations in Lac Bay, increasing from 6% to 20% in total occurrence. During the same period, native seagrass Thalassia testudinum occurrence decreased by 33%. 4. Synthesis. Our results provide first‐time evidence of large scale replacement of native seagrasses by rapidly colonising H. stipulacea in the Caribbean and add a mechanistic explanation for this invasiveness. We conclude that green turtle leaf grazing may modify the rate and spatial extent of this invasive species’ expansion, due to grazing preferences, and increased space for settlement. This work shows how large herbivores play an important but unrecognized role in species co‐existence and plant invasions of aquatic ecosystems.