Purpose The aim of this study was to evaluate the choroidal thickness in eyes with diabetic macular edema (DME) with the use of swept source optical coherence tomography (SS-OCT). Patients and methods A total of 240 eyes were included in this prospective nonrandomized study. They were divided into three groups, each including 80 eyes. Group I included patients with DME, group II included diabetic patients without retinopathy, and group III were normal persons. Fluorescein angiography was done for all diabetic patients. Macular and choroidal thicknesses were measured by SS-OCT in all eyes. Forty-five eyes from group I received two intravitreal injections of 0.5 mg/0.05 ml ranibizumab with 1-month interval. Correlation between choroidal and macular thickness was evaluated in eyes with DME. Results The mean choroidal thickness in the central subfield area was 129.7±12.5 μm in group I (eyes with DME), 200.4±2.8 μm in group II (diabetic patients without DME), and 208.3±9.7 μm in group III (normal eyes). There was a statistically significant difference in choroidal thickness between eyes with DME (group I) and both groups II and III (P<0.001), whereas there was no statistically significant difference between diabetic patients without macular edema (group II) and normal participants (group III) (P=0.2). There was no statistically significant difference among the three groups regarding age, refractive errors, and intraocular pressure. The mean best-corrected visual acuity was 0.31±0.09, 0.78±1.1, and 0.84±1.2 in groups I, II, and III, respectively (P=0.001). Negative correlation between macular and choroidal thickness was observed in eyes with DME (r=−0.7, P<0.001). There was no significant difference between choroidal thickness before and after intravitreal injection of ranibizumab (P=0.4). Conclusion Choroidal thickness measured with SS-OCT was decreased in eyes with DME in comparison with normal participants. A negative correlation was observed between choroidal and macular thickness in eyes having DME.