Background: diabetic macular edema (DME) is the most common cause of visual loss in diabetic patients. The pathogenesis of DME is multifactorial. It results from multiple biochemical and cellular changes that eventually cause leakage and exudation. A number of patient characteristics such as increasing age, females, duration of DM, poor glycemic control and moderate to severe retinopathy have been found associated with poor prognosis after cataract surgery in diabetic patients. Objective: the aim of this study was to evaluate the efficacy of phacoemulsification with posterior sub-Tenon's triamcinolone acetonide injection for prevention of progression of diabetic macular edema following uneventful cataract surgery by phacoemulsification. Patients and Methodology: this interventional study included 20 eyes of 20 diabetic patients with visually significant cataract and coexisting DME during the period from December 2017 to October 2018. Preoperative and postoperative best corrected visual acuity (BCVA) using LogMAR and intraocular pressure (IOP) were recorded. Results: statistically significant improvement in BCVA was detected when comparing postoperative values at one week, one month and three months to corresponding preoperative values. In addition, values at one month and three months were significantly improved when compared to values at one week post-operative. On other hand, values at three months revealed non-significant difference when compared to values at one month. Conclusion: this study suggests that posterior sub-Tenon's triamcinolone acetonide injection can be given safely and easily at the time of phacoemulsification surgery in patients with visually significant cataract and DME.