Ultra-high-performance fiber-reinforced concrete (UHPFRC) has recently been used in reinforced concrete (RC) full-cast or strengthened beams because it has better mechanical properties and durability. However, the main parameters that affect the flexure behavior of retrofitted RC beams with a UHPFRC layer need more study. This paper investigates experimentally and numerically the major parameters that control the effectiveness of retrofitting RC beams with an additional reinforced UHPFRC layer applied to the tensile surface. One nonstrengthened RC beam and seven retrofitted RC beams (divided into three groups) were experimentally tested under a four-point loading test. The effects of different parameters, such as the length (0.50, 0.60, and 0.80 times the length of the strengthened beams), the thickness of the UHPFRC layer (30 and 40 mm), and the retrofitting technique (with or without the concrete cover), on the crack patterns and failure modes, load–deflection behavior, load carrying capacity, energy absorption, and stiffness, were studied. The results showed that retrofitted beams exhibited significantly improved ultimate load, stiffness, and energy absorption ability compared to the control beam. UHPFRC layer length and thickness were the most significant parameters; moreover, increasing the UHPFRC layer thickness can compensate for the reduction in its length. Lastly, the validated finite element (FE) model was used to study the effect of different parameters, such as the length (0.40, 0.70, and 0.90 times the length of the strengthened beams), the thickness of the UHPFRC layer (50 mm), and the reinforced bar diameter (12 and 16 mm) in the UHPFRC layer, on the flexural behavior of the strengthened RC beams.