Vegetable fibers appear as a promising and sustainable alternative for reinforcing cementitious plates in wall-systems applications. Vegetable fibers, such as sisal, offer some advantages over synthetic ones, such as high availability, low cost, renewability, and biodegradability, in addition to exhibiting excellent physical and mechanical properties. Seeking to understand the potential application of such materials in civil construction, this study investigates the mechanical performance of cementitious plates reinforced with 5% long aligned sisal fibers. To improve the durability of the composite, 50% of the cement consumption in matrix was replaced by 40% metakaolin and 10% fly ash to avoid fiber degradation by alkaline attack. Tensile strength tests in flexion were performed in two distinct conditions, according to the guidelines of NBR 15498 (2021): saturated and in equilibrium with laboratory conditions. The plates tested in equilibrium showed an average strength of 22.31 ± 4.56 MPa, while those tested in the saturated condition recorded an average of 19.07 ± 2.68 MPa. The results highlight the potential of these sisal-reinforced cementitious plates as an effective alternative for wall-systems applications in building construction, such as internal partitions, rendered facades, eaves, among others.