Purpose: Evaluation of the effect of hyperbaric oxygen therapy (HBOT) on the progression of retinopathy, choroidal and retinal thickness in patients with type 2 diabetes mellitus (DM). Materials and methods: This prospective non-randomized cohort study consisted of 60 eyes of 30 patients who received 30 sessions of HBOT for a diabetic foot ulcer (DFU). The participants were divided into three groups; group 1: mild-moderate non-proliferative diabetic retinopathy (DRP) (n = 14), group 2: severe non-proliferative DRP (n = 20) and group 3: DRP without active proliferative findings with the applied laser for at least 2 years (n = 26). The cases were examined on base-line (measurement-1), after the 10th session of HBOT (Measurement-2), after the 20th session of HBOT (Measurement-3), after the 30th session of HBOT (Measurement-4), and after 10 days of the last session of HBOT (Measurement-5). The changes in central macular thickness (CMT; subfoveal point [CMT-SF], nasal point [CMT-N] and temporal point [CMT-T]), central choroidal thickness (CCT; subfoveal point [CCT-SF], nasal point [CCT-N] and temporal point [CCT-T]), and the stage of DRP were compared. Results: There was no significant difference between groups in terms of change ratio (CR%) in CMT-SF and CMT-N values. However, in Measurement-3, CR% in CMT-T was significantly higher in group 1 (p = 0.019). A significant increase in CMT-N and CMT-T parameters over time was observed in Group 1 (p < 0.05). There was a significant decrease in CCT-SF, CCT-N, and CCT-T values with time in each of the three groups (p < 0.05). At the end of HBOT, there was no progression or regression in the stage of DRP in any group. Conclusions: HBOT has both a thinning effect on the choroid layer in all three groups and a thickening effect on the macula in the mild-moderate non-proliferative diabetic eyes.