Periodontitis is a chronic inflammatory disease with multifactorial aetiology. The relationship between periodontal disease and systemic diseases such as diabetes, obesity, metabolic syndrome, atherosclerotic, cardiovascular disease, and cognitive disorders has been the subject of many studies. The purpose of this study was to evaluate the effectiveness of hyperbaric oxygen therapy on periodontal health in patients suffering from periodontitis and type 2 diabetes. The study was conducted with 14 patients. A total of 369 periodontal pockets in the study group and 431 in the control group were examined. For further analysis, the pockets were classified as moderately deep (4–5 mm) and deep (≥6 mm). All patients received standard non-surgical treatment: scaling and root planing (SRP). Additionally, a series of 30 hyperbaric chamber sessions was carried out in the study group. The following parameters were compared between groups: PD (probing depth), CAL (clinical attachment level), and BOP (bleeding on probing). The results of the study showed significantly better results in terms of PD reduction and CAL gain in the study group in comparison to the control group. Both groups showed a reduction in BOP (bleeding on probing) after treatment. The use of hyperbaric oxygen therapy seems to have considerable benefits in patients with type 2 diabetes.