BACKGROUND: COVID-19 is a world disaster. COVID-19 and diabetes mellitus (DM) relationship is double-faced because DM is considered one of the COVID-19 key risk factors causing an increase in COVID-19 related conditions and death rates; at the same time, high stress in patients with COVID-19 can lead to the occurrence of DM. This relationship adversely affects immune functions. Relaxation techniques have been demonstrated to positively affect immune functions, glycemic control, and stress levels. METHODS: A search in Scopus, Web of Science, Medline databases, and EBSCO has been made. The search included clinical trials and systematic and literature reviews on the effects of relaxation techniques on immune functions, glycemic control, and stress levels. RESULTS: This review found that relaxation techniques produce short-term effects on decreasing stress levels through an autonomic regulation, controlling glycemic control through decreasing hyperglycemia, and increasing immune functions through increasing immune cells and immunoglobins A. CONCLUSION: This review summarizes the important role of performing regular relaxation techniques for COVID-19 patients with DM. Relaxation techniques benefit diabetic patients with COVID-19 through decreasing stress levels, better controlling of glycemic control, and increasing immune functions.