Our study aimed to assess the associations of plasma magnesium with prediabetes and type 2 diabetes (T2D) among Chinese adults. We conducted a case-control analysis of 4447 participants: 867 newly diagnosed prediabetes patients, 1475 newly diagnosed T2D patients and 2105 normal glucose tolerance (NGT) individuals. In a subsample of 599 apparently healthy individuals, we measured plasma hs-CRP levels to examine their relation to plasma magnesium. Plasma magnesium and hs-CRP were measured by inductively coupled plasma mass spectrometry and an enzyme-linked immunosorbent assay, respectively. Plasma magnesium decreased from NGT to prediabetes to T2D, and was inversely associated with prediabetes and T2D. After adjustment for potential confounding factors, the ORs from the lowest to highest quartiles of plasma magnesium were 1, 0.57 (95% CI, 0.44-0.74), 0.49 (0.37-0.65) and 0.51 (0.37-0.70) for prediabetes, and 1, 0.26 (0.20-0.33), 0.15 (0.12-0.20) and 0.15 (0.11-0.20) for T2D. Consistently, plasma magnesium was inversely correlated with plasma hs-CRP in our subsample analysis; the geometric mean hs-CRP concentration for ascending quartiles of plasma magnesium were 1.29 (1.06-1.57), 1.16 (0.95-1.41), 1.00 (0.81-1.22), and 0.71 (0.58-0.88) mg/l. Plasma magnesium was independently and inversely associated with prediabetes and T2D in Chinese adults.