The use of sisal fiber in cementitious composites is a subject of current relevance, mostly as a result of the growing interest to make the construction industry more sustainable. However, there are also some drawbacks related to the use of sisal fibers as reinforcement, such as the low durability in alkaline matrices. Thus, this study aimed to investigate the changes in physical and mechanical characteristics of sisal fiber through fiber modifications, to guarantee improved performance as reinforcement in cement-based materials. Specifically, three different fiber modifications have been considered: the alkaline treatment, the use of a polymeric coating on the fiber surface, and a new technique of fiber surface functionalization based on polyphenol chemistry. The experimental program explored the modifications on treated fibers in terms of water absorption tendency, microstructural changes, fiber composition, tensile strength, and strain capacity. Additionally, a fundamental understanding of the interfacial properties between the fiber surface and the cementitious matrix was assessed. The investigations also included creep tests to evaluate the long-term performance of sisal fibers in aggressive conditions, considering different levels of sustained loads. It has been proven that all the proposed fiber modifications lead to improved physical, mechanical, durability-related, and time-dependent properties of sisal fibers, as well as enhancing fiber-matrix adhesion. Nonetheless, in terms of physico-mechanical performance and durability, the best results were reached by the polyphenol modified sisal fiber. After exposure to aggressive alkaline solution under loading, natural sisal remained only with 38 percent of its strength capacity, changing to approximately 55 percent for the modified fibers. The outcome of this study provided technical information to consolidate the use of sisal fibers as a reliable alternative to synthetic fibers for achieving structural cementitious composites.