This work aims to describe a facile approach for sisal fiber surface functionalization based on polyphenol chemistry, and to study fiber-matrix bonding as well as durability behavior in highly alkaline conditions. The modification process was accomplished through a two-step procedure including immersion in tannic acid (TA) solution (5 and 10 g/L) followed by treatment with octadecylamine (ODA) solution (1, 2.5, 3.75, and 5 g/L). Analytical techniques were employed to examine the effect of the treatment, including Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). SEM images along with mapping analyses were obtained to evaluate the formation, chemical composition and distribution of TA-ODA layer on modified fibers. Changes in the water uptake ability were studied by water absorption and contact angle measurements. Fiber-matrix interactions were also evaluated throughout single fiber pullout tests from a cement-based matrix. To characterize the mechanical properties of the fibers before and after the modification, single fiber direct tensile tests were applied. Moreover, the stability in alkaline environments (NaOH and pore solutions) was investigated in terms of mechanical properties degradation after fiber exposure for 28 days. The results showed that modification with a low concentration of TA and ODA solutions was efficient to reduce the hydrophilic tendency of these fibers while enhancing roughness and interlock with the cement-based matrix. Also, the modified fiber presented improved stability in aggressive alkaline environment. This work demonstrates that polyphenol chemistry could be a simple and effective sisal fiber modification alternative to enhance fiber stability and interfacial bond with cement-based matrices.