Introduction Adult-onset type 2 diabetes mellitus (T2DM) is defined as a chronic hyperglycemic state, characterized by insulin resistance and declining islet B-cell function, eventually leading to islet B-cell function failure. The present study evaluated the association of T2DM with the type of blood group. Methodology A case-control study was conducted from April 2020 to September 2021 in Karachi, Pakistan. An electronic questionnaire was used to determine if there is an association between ABO blood groups and type 2 diabetes mellitus. Our study involved two groups with an equal number of participants. The patient group contained participants who had been diagnosed with type 2 diabetes mellitus, whereas the control group contained participants who had never been diagnosed with type 2 diabetes mellitus. Information was collected through a self-administered electronic questionnaire which was circulated through social media. Results The mean ± SD age was reported to be 25.98 ± 12 years. The study found a significant association between blood group B and type 2 diabetes mellitus (p=0.006), whereas a negative association was seen between the blood group O and type 2 diabetes mellitus (p=0.001). It should be noted, however, no significant association was found between the blood groups A and AB and type 2 diabetes mellitus (p>0.05). Conclusion The results of this study indicate that there is an association between type 2 diabetes mellitus and the ABO blood group system; a significant association was found between blood group B and risk of type 2 diabetes mellitus (T2DM). Nevertheless, we recommend regular screening for T2DM in individuals with a high-risk profile. Those at risk can adopt measures that are beneficial for them in the long run such as dietary control and physical exercise. Further studies using explorative techniques with a diversified population are recommended.