Incorporating natural and synthetic fibers, with regards to their specific characteristics and properties, is a foundational strategy researchers employ to enhance performance and overcome the shortcomings of concrete. The application of fibers for the internal curing of high-performance concrete is a notable area of investigation within this realm. The minimal water content and substantial volume of cementitious materials in high-performance concrete underline the critical role of effective curing in attaining superior strength and durability for such concrete. Accordingly, this study examined the physical and mechanical properties, including density, water absorption, strain at failure and tensile strength, of natural date palm (Sis) fibers, to assess their efficacy as internal curing agents in high-performance concrete. The notable absorption capacity (127.5%) and the ability of these fibers to maintain the absorbed moisture effectively counteract moisture loss in capillary pores, thereby facilitating optimal curing conditions. These fibers were subsequently utilized in varying percentages and lengths to develop mix designs for high-performance concrete. Tensile and compressive samples were fabricated per ASTM standards and stored in different conditions such as submerged in water ponds, in ambient room conditions, and wrapped in wet gunny and covered with plastic. This research findings reveal that an increase in fiber percentage leads to a decrease in compressive strength, with the most pronounced decline observed with 3% long fibers, by approximately 23%. Conversely, introducing these fibers contributed to an enhancement in the concrete's tensile strength, the most notable improvement was recorded in the mixture with 2.5% long fibers, by approximately 3.5%.