We present the first experimental evidence for the ternary complexation of calcium and magnesium ions with plutonyl(VI)tricarbonate species in carbonate-containing aqueous solutions using visible-NIR spectrophotometric titration. Prior to studying the ternary plutonyl(VI) carbonate complexation, visible-NIR absorption spectral information of PuO₂(CO₃)₂²⁻ and PuO₂(CO₃)₃⁴⁻ was successfully obtained. PuO₂(CO₃)₂²⁻ has a prominent peak at 853 nm and its molar absorptivity was determined to be ε_{853, PuO2(CO3)2²⁻} = 49.0 ± 4.2 M⁻¹·cm⁻¹. The spectrophotometric titration results by adding calcium or magnesium to the plutonyl(VI) carbonate system consisting of PuO₂(CO₃)₂²⁻ and PuO₂(CO₃)₃⁴⁻ indicate the formation of CaPuO₂(CO₃)₃²⁻ and MgPuO₂(CO₃)₃²⁻ complexes and provide the formation constants at 0.1 M H/NaClO₄ for MPuO₂(CO₃)₃²⁻ from PuO₂(CO₃)₃⁴⁻, log K = 4.33 ± 0.50 and 2.58 ± 0.18 for M = Ca²⁺ and Mg²⁺, respectively. In addition, the formation constants of CaPuO₂(CO₃)₃²⁻ and MgPuO₂(CO₃)₃²⁻ from PuO₂(CO₃)₃⁴⁻ at infinite dilution (log K°) were proposed to be 6.05 ± 0.50 and 4.29 ± 0.18, respectively, based on the correction of ionic strength using the Davies equation. The absorption spectrum of the ternary plutonyl(VI) complexes of CaPuO₂(CO₃)₃²⁻ is similar to that of PuO₂(CO₃)₃⁴⁻ with the exception of a characteristic absorption peak at 808 nm (ε_{808, CaPuO2(CO3)3²⁻} = 42.9 ± 1.6 M⁻¹·cm⁻¹). According to the calculated aqueous plutonyl(VI) speciation including the ternary plutonyl(VI) complexes, CaPuO₂(CO₃)₃²⁻ is considered the dominant Pu(VI) species under environmental conditions, and plutonyl(VI) may be more mobile than expected in previous assessments.