Prestressing steel is an effective means of increasing the load carrying capacity and control the deflection of structural steel members. Although the method of prestressing is not new, the application and development of the technique for steel members has been limited and not as advanced as prestressed concrete structures. Prestressing force is commonly introduced using hydraulic jacks and utilizes high strength tendons as the prestressing material. In this paper, a new method of applying prestressing force is introduced. The method involves a segmental beam consisting of three segments and a normal strength bottom plate (BP) as the pretressing material attached to the two initially inclined outer segments of the steel H-beam. Pretressing force is introduced by taking advantage of the geometry of the segmental assembly of the beam. Experiment, analytical studies, and finite element (FE) analysis using ABAQUS is performed to verify the feasibility and applicability of the method and to understand the behavior of a mechanically prestressed segmental steel beam. Results showed that the prestressing method significantly increases the load carrying capacity and at the same time reduces the deflection of the beam. The difficulties encountered in the method are the rigorous assembly and the introduction of the exact required amount of prestressing force. Overall, the method of prestressing performed in the study can be used to introduce pretressing force which improves the performance of steel beams, provided that proper detailing is implemented.