The purpose of the study is to compare single injection of intravitreal diclofenac (IVD) with intravitreal bevacizumab (IVB) in the treatment of eyes with naïve diabetic macular edema (DME). In this randomized clinical trial, 57 eyes of 57 patients were randomly assigned to IVD group (30 eyes), cases who received a single intravitreal injection of diclofenac (500 μg/0.1 ml), and IVB group (27 eyes), cases who received a single intravitreal injection of bevacizumab (1.25 mg). Change in best-corrected visual acuity in logMAR at week 12 was the primary outcome measure. Secondary outcomes included changes in central macular thickness, macular leakage, and potential injection-related complications. Best-corrected visual acuity improved significantly more in the IVD group than in the IVB group (P = 0.033), from 0.57 ± 0.25 to 0.49 ± 0.31 versus 0.55 ± 0.24-0.59 ± 0.27 logMAR at 12 weeks, respectively. However, the difference of macular thickness changes was in favor of IVB, but not to a significant level. The amount of change in leakage was not significantly different between the groups either. None of the eyes, in either group, developed ocular hypertension (≥23 mmHg) or cataract progression. No important injection-related complication was observed during the study period. This study demonstrated the superiority of IVD over IVB in the treatment of naïve DME regarding functional, but not anatomical outcomes. Therefore, using IVD as an adjunct or even alternative to other treatments might enhance the functional outcomes in such cases. Further studies are warranted to confirm potential benefit of IVD observed in this study.