CONTEXT: Low magnesium levels, which are common in people with type 2 diabetes, are associated with increased levels of proinflammatory molecules. It is unknown whether magnesium supplementation decreases this low-grade inflammation in people with type 2 diabetes. OBJECTIVE: We performed multidimensional immunophenotyping to better understand the effect of magnesium supplementation on the immune system of people with type 2 diabetes and low magnesium levels. METHODS: Using a randomized, double-blind, placebo-controlled, 2-period, crossover study, we compared the effect of magnesium supplementation (15 mmol/day) with placebo on the immunophenotype, including whole blood immune cell counts, T-cell and CD14+ monocyte function after ex vivo stimulation, and the circulating inflammatory proteome. RESULTS: We included 12 adults with insulin-treated type 2 diabetes (7 males, mean ± SD age 67 ± 7 years, body mass index 31 ± 5 kg/m2, HbA1c 7.5 ± 0.9%) and low magnesium levels (0.73 ± 0.05 mmol/L). Magnesium treatment significantly increased serum magnesium and urinary magnesium excretion compared with placebo. Interferon-γ production from phorbol myristate acetate/ionomycin stimulated CD8+ T-cells and T-helper 1 cells, as well as interleukin (IL) 4/IL5/IL13 production from T-helper 2 cells was lower after treatment with magnesium compared with placebo. Magnesium supplementation did not affect immune cell numbers, ex vivo monocyte function, and circulating inflammatory proteins, although we found a tendency for lower high sensitivity C-reactive protein levels after magnesium supplementation compared with placebo. CONCLUSION: In conclusion, magnesium supplementation modulates the function of CD4+ and CD8+ T-cells in people with type 2 diabetes and low serum magnesium levels.