Post-stroke depression (PSD) is a common psychiatric complication of stroke that is associated with a poor outcome in stroke patients. Our aim was to assess the association between the serum magnesium levels and the presence of PSD in Chinese patients. Two hundred nine stroke patients were included in the study. Depressive symptoms were measured by the 17-Hamilton Rating Scale for Depression at 3 months after stroke. Based on the depressive symptoms, diagnoses of depression were made in line with the DSM-IV criteria for PSD. Serum magnesium levels were evaluated using the dimethyl aniline blue colorimetric method at admission. Multivariate analyses were conducted using logistic regression models. Further, 120 normal subjects were recruited, and their serum magnesium levels were also measured as control. At 3 months, fifty-nine patients (28.2%) were diagnosed as PSD. The serum magnesium levels were significantly lower in both PSD patients and non-PSD patients than in normal subjects (p < 0.001). Indeed, patients with PSD showed lower serum magnesium levels (p < 0.001) than did non-PSD patients at admission. In the multivariate analyses, after adjusting for potential variables, we found that an increased risk of PSD was associated with serum magnesium levels <= 0.84mmol/L (OR 2.614, 95% CI 1.178-5.798, p=0.018). Low serum magnesium levels at admission were found to be associated with the presence of PSD at 3 months after stroke.