Zinc inhibits vascular calcification in vivo and in vitro. Patients with type 2 diabetes mellitus show hypozincemia and are at an elevated risk of cardiovascular events. Recently, an in vitro test (T50-test) was developed for determination of serum calcification propensity and a shorter T50 means a higher calcification propensity. This cross-sectional study investigated the association between serum zinc and T50 in 132 type 2 diabetes mellitus patients with various kidney functions. Furthermore, the effect of exogenous zinc on T50 was also investigated in vitro using separately pooled serum samples obtained from healthy volunteers and patients with hemodialysis. We measured T50 levels using the established nephelometric method. The median (interquartile range) levels of T50 and serum zinc were 306 (269 to 332) min, and 80.0 (70.1 to 89.8) µg/dL, respectively. Serum zinc level showed a weak, but positive correlation with T50 (rs = 0.219, p = 0.012). This association remained significant in multivariable-adjusted analysis, and was independent of known factors including phosphate, calcium, and magnesium. Kidney function and glycemic control were not significantly associated with T50. Finally, in vitro experiments showed that addition of a physiological concentration of exogenous zinc chloride significantly increased serum T50. Our results indicate that serum zinc is an independent factor with a potential role in suppressing calcification propensity in serum.