Background: Topical glyceryl trinitrate therapy has previously demonstrated short-term pain reduction in patients with supraspinatus tendinopathy. Hypothesis: Topical glyceryl trinitrate improves outcome measures in patients with supraspinatus tendinopathy. Study Design: Randomized controlled clinical trial; Level of evidence, 1. Methods: Fifty-three patients (57 shoulders) were recruited, and the authors completed a prospective, randomized, double-blinded, placebo-controlled clinical trial of continuous topical glyceryl trinitrate treatment (1.25 mg/24-h glyceryl trinitrate). Results: The glyceryl trinitrate group had significantly reduced shoulder pain with activity, at night, and at rest at week 24 (P = .03); reduced internal rotation impingement at week 24 (P = .02); increased range of motion in abduction and internal rotation at week 24 (P = .04); and increased force at weeks 12 and 24 with supraspinatus muscle testing (P = .001), external rotation (P = .04), internal rotation (P = .01), adduction (P = .04), and subscapularis pushoff (P = .01). Forty-six percent of patients on glyceryl trinitrate patches were asymptomatic with activities of daily living at 6 months compared with 24% of patients with tendon rehabilitation alone (P = .007, χ 2 analysis). Mean effect size for all outcome measures was 0.26. Discussion: Topical glyceryl trinitrate treatment significantly improved pain scores, range of motion, internal rotation impingement, muscular force, and patient outcomes in patients with supraspinatus tendinopathy. Topical glyceryl trinitrate should be included as part of nonsurgical management of chronic tendinopathies.