Depression is a common and disabling condition, and there has long been interest in whether vitamin D-a retroactive steroid hormone with receptors throughout the brain-plays a part in mood regulation. This narrative review brings together observational and Interventional evidence on two related questions: whether low vitamin D status is associated with depression, and whether vitamin D supplementation reduces depressive symptoms in adults. Drawing on major meta-analyses and representative primary studies identified through database searches, the review finds a consistent association between lower serum 25-hydroxyvitamin D and a higher likelihood and severity of depression, with reported effects ranging from a small-to-moderate standardized mean difference to a more than two-fold hazard in prospective cohorts. The evidence for supplementation is more mixed. Trials in UN-selected or vitamin D–replete adults have generally shown little benefit, whereas pooled analyses restricted to people with clinically diagnosed major depression report clear symptom improvement. Differences in dose, treatment duration, baseline vitamin D status, and clinical population appear to explain much of this inconsistency. Taken together, the association between vitamin D deficiency and depression is robust, but causation remains unproven. On current evidence, correcting deficiency is best viewed as a low-risk adjunct for depressed patients who are also deficient rather than a general antidepressant. Better-targeted randomized trials are needed to establish whether restoring vitamin D genuinely improves mood.