Background: Numerous reports have been published describing the presence of diabetes mellitus (DM) in COVID-19 patients, indicating a connection between the two conditions. In order to determine the clinical manifestations and outcomes in this patient cohort as well as to summarise the prevalence of DM in COVID-19 patients, we conducted this systematic review. Materials and methods: The electronic MEDLINE, Embase, Cochrane, and PubMed databases were searched. To make the search procedure more sensitive while finding research, the Medical Subject Heading (MeSH) terms were developed. Some of these words included "Diabetes Mellitus", "COVID-19," and "Association". Results: The qualitative analysis includes six studies. Studies that were cited provided information on both adult and paediatric patients. Between the studied populations, the investigated results varied greatly. Conclusions: Inclusion studies varied in how they reported the COVID-19 outcomes for length of hospital stay, hospitalisation, ICU admission, DKA rate, and severe hypoglycemia. In order to draw a firm conclusion, additional research is still necessary due to the varied study populations and the presence of numerous restrictions.