Background The inflammatory response associated with acute pulmonary embolism (PE) contributes to the development of right ventricular (RV) dysfunction. Nonsteroidal anti-inflammatory drugs (NSAIDs) may facilitate the reversal of PE-associated RV dysfunction. Methods We randomly assigned normotensive patients who had acute PE associated with echocardiographic RV dysfunction and normal systemic blood pressure to receive intravenous (IV) diclofenac (two doses of 75 mg in the first 24 h after diagnosis) or IV placebo. All patients received standard anticoagulation with subcutaneous low-molecular-weight heparin (LMWH) and an oral vitamin K antagonist. RV dysfunction was defined by the presence of, at least, two of the following criteria: i) RV diastolic diameter > 30 mm in the parasternal window; ii) RV diameter > left ventricle diameter in the apical or subcostal space; iii) RV free wall hypokinesis; and iv) estimated pulmonary artery systolic pressure > 30 mm Hg. Persistence of RV dysfunction at 48 h and 7 days after randomization were the primary and secondary efficacy outcomes, respectively. The primary safety outcome was major bleeding within 7 days after randomization. Results Of the 34 patients randomly assigned to diclofenac or placebo, the intention-to-treat analysis showed persistent RV dysfunction at 48 h in 59% (95% confidence interval [CI], 33–82%) of the diclofenac group and in 76% (95% CI, 50–93%) of the placebo group (difference in risk [diclofenac minus standard anticoagulation], − 17 percentage points; 95% CI, − 47 to 17). Similar proportions (35%) of patients in the diclofenac and placebo groups had persistent RV dysfunction at 7 days. Major bleeding occurred in none of patients in the diclofenac group and in 5.9% (95% CI, 0.2–29%) of patient in the placebo group. Conclusions Due to slow recruitment, our study is inconclusive as to a potential benefit of diclofenac over placebo to reverse RV dysfunction in normotensive patients with acute PE. Clinical Trial Registration: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01590342.