Background: Major Depressive Disorder (MDD) is a disabling psychiatric condition increasingly linked to metabolic and inflammatory disturbances. Alterations in serum calcium and albumin may contribute to neurobiological mechanisms underlying depression. Objective: To assess and compare serum total calcium and albumin levels between patients with Major Depressive Disorder and healthy controls. Materials and Methods: This cross-sectional case–control study was conducted at a tertiary psychiatric facility in Maiduguri, Nigeria, from January to June 2025. A total of 120 outpatients diagnosed with MDD using DSM-5 criteria and 120 age- and sex-matched healthy controls were enrolled. Serum total calcium was measured using the Arsenazo III method, while albumin was determined using the bromocresol green assay. Multivariate logistic regression analysis was applied. Results: Mean serum calcium (8.4 ± 0.6 vs. 9.2 ± 0.5 mg/dL) and albumin (3.6 ± 0.4 vs. 4.2 ± 0.3 g/dL) levels were significantly lower in MDD patients compared with controls (p < 0.001). Increasing serum calcium and albumin concentrations were independently associated with reduced odds of MDD. Conclusion: Lower serum calcium and albumin levels are significantly associated with Major Depressive Disorder, suggesting their relevance in depression pathophysiology.