Activated alumina (AA) with metastable crystal phase and porous structure has shown great potential for fluoride ion removal. Conventional AA is obtained by direct calcination, which limits its fluoride adsorption ability due to the low surface area and limited active sites. In this work, a highly porous AA was synthesized by a sol–gel method, followed with supercritical drying (including ethanol supercritical drying and carbon dioxide supercritical drying) and calcination. The fluoride ion adsorption properties of AA were studied at different concentrations of fluoride in solution. The Langmuir model, Freundlich model, pseudo-first-order model and pseudo-second-order model were used to describe the adsorption process. The results show that the resulting AAs possess a high porosity and a high specific surface area (up to 660 m2/g). The fluoride adsorption capacity of AA increases with the increase of specific surface area and porosity (up to 119.2 mg/g), which is much higher than that of AA prepared by the traditional method (direct calcination). Moreover, all the fitted correlation coefficients of the four models exceed 90%, indicating both chemical and physical adsorption in AA.