Purpose: Extensor carpi ulnaris (ECU) subsheath injuries result in ulnar-sided wrist pain and often present concurrently with intrinsic ECU pathology and ulnocarpal compartment injuries. There is a lack of surgical outcome data despite the variety of described ECU subsheath pathologies and reconstructive strategies. Methods: We retrospectively reviewed our hand-center experience of 33 patients who prospectively underwent radially based extensor retinacular sling ECU subsheath reconstruction by 4 hand surgery-fellowship-trained surgeons between April 2010 and April 2021. Preoperative clinical and magnetic resonance imaging findings, along with intraoperative findings, were cataloged. Statistical analysis was conducted via a 2-tailed paired t test. Results: The median age at the time of surgery was 44 years (range, 18-63 years). Twenty (60.6%) patients underwent reconstruction on their dominant wrist. The median time between symptom onset and surgery was 6.5 months (range, 4 days-16.1 years). Eight (18%) patients were collegiate-level or professional athletes. Ten (30.3%) patients had frank ECU snapping on the preoperative examination with no recurrence or apprehension on the postoperative examination. All 33 patients underwent a preoperative magnetic resonance imaging. Fifteen (45.4%) patients had intrinsic ECU tendinopathy, 19 (57.6%) patients had ECU tenosynovitis, 18 (54.5%) patients had triangular fibrocartilage complex tears, 20 (60.6%) patients had ulnocarpal synovitis, and 2 (6.1%) patients had lunotriquetral interosseous ligament tears. The mean postoperative pain on a visual analog scale was 0.39 ± 0.55. Grip strength, wrist flexion-extension, and pronosupination arcs (P < .05) showed excellent recovery after surgery. The mean time to unrestricted return to sports was 97.3 ± 19.7 days for the athletes in this study. There were no major complications. Conclusions: Radially based extensor retinacular sling ECU subsheath reconstruction resulted in satisfactory improvements in range of motion and grip strength. Although the mean improvements in these parameters were statistically significant, the clinical significance of these postoperative improvements remains to be defined. Type of study/level of evidence: Therapeutic, Level IV.