Diabetes mellitus is defined as a group of chronic metabolic diseases that are characterized by chronic hyperglycemia and other related metabolic disturbances. It is caused either by relative/absolute insulin deficiency or cellular resistance to insulin action, or both. Due to delay in the diagnosis process, and fear of the disease and its investigative procedure by some patients, diabetes mellitus has become the major cause of death. The most commonly employed investigative procedure to diagnose diabetes mellitus and controlling glycemia is blood investigation. Unfortunately, the procedure is invasive, painful, and may cause discomfort to patients due to the need for frequent testing. Consequently, a noninvasive, much simpler, and painless procedure is very desirable. Saliva represents an attractive alternative sample and offers a distinct advantage as it can be collected noninvasively and easily without special skill, and is low cost. The present review has found more studies with a positive correlation between blood and salivary glucose concentrations than those with a negative correlation. The difference between these studies' findings may be attributed to the difference in study population and criteria of selection, samples (saliva and blood) collecting methods, analyzing methods, and influencing factors that should be considered before the test.