The purpose of this study was to determine the morphological and functional changes in the macula after pars plana vitrectomy with the creation of a posterior vitreous detachment in eyes with diabetic macular edema (DME). A simple posterior vitreous detachment was created during pars plana vitrectomy accompanied by simultaneous cataract surgery in 19 eyes of 17 patients with DME. The visual acuity (log MAR units), multifocal electroretinograms (mfERGs), and optical coherence tomographically (OCT)-determined foveal thickness were determined preoperatively and at 6 months postoperatively. The median preoperative log MAR visual acuity was 0.7 (20/100; range, 0.3–1.1), and it improved significantly to 0.4 (20/50; range, 0.15–1.0) postoperatively (p = 0.016). The median foveal thickness was 510 (range, 194–800) m preoperatively and was significantly reduced to 201 (range, 60–631) m postoperatively (p =0.0002). The median of response density of the mfERGs in the macular area was 6.3 nV/deg² (range, 2.8–14.8) preoperatively, and 6.1 nV/deg² (range, 2.4–14.3) postoperatively, a non-significant change (p = 0.27). The median of peak latency of the macular response was 30.0 ms (range, 25.0–44.2) preoperatively and it decreased significantly to 28.3 ms (range, 26.7–35.0) postoperatively (p = 0.003). In conclusion, vitrectomy with the creation of a PVD in eyes with DME improved the visual acuity and foveal thickness significantly. In addition, a decrease of the peak latency of the macular mfERGs suggested an improvement of the physiological function of the macula although the amplitude of the mfERGs was not changed significantly.