New-onset diabetes mellitus (DM) has been lately observed among patients with post-COVID-19 syndrome. ACE2 and TMPRSS22 receptors were found expressed in the pancreas, thus acting as an entry gate leading to infection-induced pancreatic injury. Moreover, COVID-19 and DM have been reported to interact bidirectionally, in which existing DM comorbidity increases the patient's chance of getting severe COVID-19 and vice versa. Hyperbaric oxygen therapy (HBOT) is one of the therapy modalities given by applying administration of oxygen 100% under pressure of more than 1.4 ATA. HBOT has been shown to improve cellular function such as decreasing oxidative stress, inflammation, and mitochondrial dysfunction. These protective effects might correlate with preserved pancreatic cells, which improved insulin homeostasis. However, the potency and mechanism of HBOT to these patients remain unclear. Hence, we conduct a review of the available evidence concerning the potential mechanisms of HBOT in improving new-onset DM among post-COVID-19 syndrome patients. Current literature showed that HBOT might be beneficial for these patients, thus this modality might be a new breakthrough for researchers and health workers considering post-COVID-19 syndrome incidence tends to rise as the pandemic grows.