The retrospective study included 496 hospitalized postmenopausal women from January 1, 2021, to June 30, 2025, of whom 288 had osteoporosis (T-score ≤ -2.5) and 208 did not. Patients with osteoporosis were older, had lower body mass index (BMI), lower bone mineral density (BMD), and lower serum levels of magnesium (Mg) and phosphorus (P). ROC analysis showed moderate discriminatory power of Mg for osteoporosis (AUC 0.602, 95% CI 0.551–0.652) with an optimal cutoff of 0.845 mmol/L. After adjustment, Mg ≤ 0.845 mmol/L was independently associated with a higher risk of osteoporosis (OR 1.940, 95% CI 1.312–2.868; P < 0.05), as were β-CTx (OR 2.229), age (OR 1.067), and BMI (OR 0.894). 31 patients with osteoporotic fractures had lower levels of Mg and P compared to 465 without fractures. The nomogram with BMI, age, Mg, P, β-CTx and P1NP showed acceptable calibration and clinical benefit. Lower serum Mg levels are associated with osteoporosis and fractures in these women.