Magnesium (Mg) is essential for bone formation and as a co-factor of more than 300 enzymatic reactions. In pigs, effects of Mg and fiber content of the diet on Mg digestive absorption has been little studied. In two replicates, 60 piglets (10/diet), weighing 40 kg at the beginning of the trial, were placed in individual cages for 16 days and fed three standard diets (13% NDF) with 0.15%, 0.30%, and 0.60% Mg, respectively, and three high-fiber diets (19% NDF) with 0.15%, 0.30%, and 0.60% Mg, respectively, all iso-NE and iso-protein, with a totally randomized design. Feces and urine were collected during the last 7 days of each replicate, and their Mg contents were analyzed. After slaughter, Mg contents of metacarpals and plasma were also determined. Diets containing 0.60% Mg caused diarrhea and were removed from the experiment. The high-fiber diets reduced digestibility of dry matter and Mg (P < 0.01). As dietary Mg content increased, body retention of Mg clearly increased with standard diets but decreased with high-fiber diets (P < 0.001). Plasma and metacarpal Mg contents increased with increased dietary Mg content. The increase in metacarpal Mg content was higher with standard than with high-fiber diets. Plasma Mg can serve as an indicator of Mg retention for standard diets but not for high-fiber diets. Diets did not affect growth performance significantly.