This study examined the effects of dietary magnesium (Mg) supplementation on bone turnover and serum parathyroid hormone (PTH) levels in rats fed a high-phosphorus (P) diet. Male rats were randomized by weight into three groups, and fed a control diet (control), a high-P diet (HP) or a high-P and high-Mg diet (HPHMg) for 14 days. Serum osteocalcin levels were significantly higher in the HP and HPHMg groups than in the control group. Serum CTx levels were significantly higher in the HP and HPHMg groups than in the control group, while the levels in the HPHMg group were significantly lower than in the HP group. Serum PTH levels were significantly higher in the HP group than in the control and HPHMg groups. Dietary Mg supplementation had a significant influence on serum PTH levels in the HP and HPHMg groups. These results suggest that dietary Mg supplementation suppresses the high bone resorption induced by a high-P diet via inhibition of PTH secretion. Moreover, our results suggest that dietary Mg supplementation may be beneficial for the prevention of bone loss with high-P diet administration.