Purpose To evaluate the safety and efficacy of intravitreal injection of bevacizumab before vitrectomy in advanced proliferative diabetic retinopathy. Methods A randomized clinical trial was performed on 40 eyes of 40 patients. Inclusion criteria were advanced proliferative diabetic retinopathy with fractional retinal detachment and HbA1c <7%. Patients were randomly assigned into 2 groups. Patients in one group had an intravitreal injection (1.25 mg) of bevacizumab 48 hours before 23-G pars plana vitrectomy surgery was performed, whereas the other group did not. Best-corrected visual acuity, intraocular pressure, and fundus photographs were taken prior to surgery 1 week and 3 and 6 months postoperatively. Results Effective vitrectomy time was 8.05 minutes in the bevacizumab group vs 16.8 minutes in the non-bevacizumab group. Statistically significant differences were observed in visual acuity at 1 week and 3 and 6 months follow-up between the 2 groups (p<0.05 for each visit). Also, there was less bleeding intraoperatively in the bevacizumab group. Mean final visual acuity in the bevacizumab group was 0.82 logMAR and 2.01 logMAR in the non-bevacizumab group. Conclusions Adjuvant intravitreal injection of bevacizumab prior to vitrectomy in diabetic retinopathy with fractional retinal detachment significantly eases the procedure, diminishing intraoperative complications, and leads to a better visual outcome.