Four-point bending tests were conducted on precast ultra-high-performance concrete (UHPC) segmental beams reinforced with unbonded prestressing tendons. A nonlinear finite element model was established and rigorously validated against the experimental data to simulate their flexural behavior. The experimental results show that compared with monolithic beams, the segmental beams experience a slight reduction in flexural capacity of 9.22% and 12.44% for the double-joint and triple-joint configurations, respectively. Nevertheless, the segmental beams possess greater ductility reserves; specifically, their average peak displacements increased from 9.83 mm for the monolithic beams to 11.60 mm and 14.78 mm for the double-joint and triple-joint beams, respectively, demonstrating substantially improved ductility. Based on the validated finite element model, extensive parametric analyses were performed. The numerical results indicate that concrete strength and steel strand reinforcement ratio significantly enhance the load-carrying capacity. Furthermore, shifting the joint positions away from the loading points increases the beam’s bending capacity, though this enhancement aggressively flattens out beyond a critical distance threshold of 0.25 L (L is the effective span). Finally, segmental beams with shear-resistant keyed joints exhibit higher overall stiffness and ultimate load-carrying capacity compared to those with plain flat joints.