OBJECTIVE Population-based studies have revealed a high prevalence of cognitive impairment after stroke. We aimed to determine the impact of serum magnesium (Mg2+) levels on the occurrence of poststroke cognitive impairment (PSCI). METHODS Acute ischemic stroke patients (n = 327) were enrolled in our study and serum Mg2+ levels were assessed on admission. The cognitive performance of each patient was evaluated using the Mini-Mental State Examination (MMSE) at a 1-month follow-up visit. RESULTS One hundred five (32.1%) patients were diagnosed with PSCI at 1-month poststroke. The serum Mg2+ levels in both the PSCI group and the non-PSCI group were significantly lower than those in normal control group (P<0.001). In addition, the PSCI group had lower levels of serum Mg2+ compared to the non-PSCI group (P=0.003). In the binary logistic regression analysis, a serum Mg2+ level of ≤0.82 mmol/L was significantly associated with an increased risk of developing PSCI by the 1-month follow-up (OR 2.236, 95% CI 1.232-4.058, P=0.008), as was age (OR 1.043, 95% CI 1.014-1.073, P=0.003). CONCLUSIONS Our results demonstrate the existence of a significant association between low levels of serum Mg2+ and the occurrence of PSCI 1-month poststroke, and these results suggest that low levels of serum Mg2+ on admission may serve as a risk factor for developing PSCI by 1-month poststroke.