<p>This study experimentally and numerically investigated the shear behavior of UHPFRC deep beams with internal-opening reinforcement, focusing on how internal-opening reinforcement detailing can mitigate opening-induced shear capacity loss. The test program included eight simply supported UHPFRC deep beams tested under four-point loading and arranged into two groups with different geometries. Group I comprised five beams with an <i>a/d</i> ratio of 0.61, including one solid reference beam and four beams with a square opening within the shear span, and intersected the load path. Group II comprised three beams with an <i>a/d</i> ratio of 0.79 and different opening sizes. Three internal opening reinforcement techniques with different ratios were examined: (1) additional vertical and horizontal bars around the opening (<i>µ</i>_<i>av</i> = <i>µ</i>_<i>ah</i> = 2.7% or 2.4%), (2) additional stirrups (<i>ρ</i>_<i>va</i> = 4.2% or 5.6%) with vertical and horizontal bars around the opening (<i>µ</i>_<i>av</i> = <i>µ</i>_<i>ah</i> = 2.7% or 2.4%), and (3) diagonal cross-bars around the opening (<i>µ</i>_<i>ax</i> = 1.8%). The results showed that the unreinforced opening reduced the cracking load by 41.4% and the ultimate load by 40.4%, compared with the solid beam. Techniques (1), (2), and (3) increased ultimate shear capacity by 16.5%, 69.2%, and 25.3%, respectively, versus the unreinforced opening beam, achieving approximately 54.6%, 79.4%, and 58.8% recovery of the solid-beam capacity. A 3D numerical model using concrete damage plasticity reproduced damage and load–deflection responses, with mean experimental-to-numerical ratios of 1.08 for ultimate shear load and 1.30 for midspan deflection, supporting the model’s predictive reliability for deep beams with internal opening reinforcement.</p>