The aim of the present prospective controlled trial was to evaluate the adjuvant role of posterior subtenon triamcinolone in the treatment of diffuse diabetic macular edema (DDME). Thirty one (31) eyes with DDME were divided into an interventional group subjected to grid laser photocoagulation and 0.5 mL of 40 mg/mL posterior subtenon triamcinolone, and a noninterventional group subjected to grid laser photocoagulation only. Best-corrected visual acuity, contrast sensitivity, and intraocular pressures were assessed at presentation, and at 6, 12, and 24 weeks after intervention. The interventional group showed a statistically significant (p < 0.05) change in both mean best corrected visual acuity, from 20/160 to 20/100 (two-line increase on Early Treatment Diabetic Retinopathy Study (ETDRS)) (p = 0.024), and mean contrast sensitivity, from 0.9 to 1.09 (p = 0.01), at the end of 24 weeks. A significant rise in intraocular pressure was not described in any of the patients in either group. Posterior subtenon triamcinolone injection is a useful and safe adjunct to the conventional management of diabetic macular edema.