Major Depressive Disorder (MDD) is significant public health concern that is often associated with an increased risk of suicide attempts. Emerging research suggests that Vitamin D3, a nutrient primarily obtained through sunlight exposure and certain foods, plays a crucial role in brain health and mood regulation and thus affects various psychiatric disorders, including depression. Objective: To determine the association between decreased Vitamin-D levels and a history of attempted suicide among patients suffering from Major Depressive Disorder. Methods: A comparative cross-sectional study was carried out from December 2020 to June 2021, on a sample of 104 patients diagnosed with Major Depressive Disorder. Half of the samples were with Vitamin-D3 deficiency while half of the sample were without Vitamin-D3 deficiency. This research was carried out at the Department of Psychiatry and Behavioral Sciences, Liaquat University Hospital, Hyderabad, Pakistan. Data were analyzed with SPSS version 21.0. Results: The mean age of the sample stood at 44 years (09 ± SD). A majority of the sample comprised of male (55.77%), hailing from an urban background and a middle socioeconomic set-up (60.58%). The mean Vitamin-D level was 18.63 ng/ml, while the mean Vitamin-D3 level was 14.77 ng/ml and 22.05 ng/ml for groups A and B respectively. History of attempted suicide was more prevalent among patients with Vitamin D3 deficiency. Conclusion: The research concluded that Vitamin-D deficiency is found to be one of the factor for suicidal attempts in patients with Major Depressive Disorder.