Background: The association between diabetes mellitus and dementia is not well-established such that found between diabetes mellitus and vascular diseases. This article aimed at reviewing the longitudinal prospective studies which evaluated the association between diabetes mellitus and incidence of dementia among different age groups of patients. Methods: An electronic search was performed by Google Scholar, PubMed, and Sciencedirect to identify all relevant articles. The number of eligible articles based on titles and abstracts were 21 eligible articles. After that, the irrelevant, duplicated and other reviews studies were excluded based on, which resulted in the exclusion of 13 irrelevant articles. Finally, 7 articles were included in this review. The data collected about mean age of the population, type and duration of diabetes, the strength of association and the statistical significance. Results: The strength of association between dementia and diabetes mellitus varied in the included studies, the reported ratios varied from a relative risk of 1.2 to a 4.77. Regarding the statistical significance of these associations, all the studied articles revealed significant associations with P values less than 0.05. Conclusion: This review supported the evidence of the association between diabetes mellitus and occurrence of dementia depending on the findings of the recent epidemiological studies.