The aim of this work is to determine the solubility product of plutonium hydroxide under reducing conditions and to ascertain the stability of Pu(OH)₃ (am) in water. Hydrolysis of Pu(III) and solubility of Pu(OH)₃ (am) were investigated at a constant ionic strength of 0.1 M NaClO₄. Coulometric titration was adopted to adjust the pH of plutonium solutions, during which the electrolytic reducing conditions maintained the oxidation state of Pu(III). Chemical speciation for dissolved plutonium was investigated using sensitive spectrophotometry coupled with a liquid waveguide capillary cell. The spectroscopic investigations indicated that dissolved Pu(IV), Pu(V), and Pu(VI) species were ignorable under these experimental conditions. The absorbance of Pu³⁺ ions decreased due to hydrolysis of Pu(III) but the absorbance of Pu(III) hydrolysis species was not distinguishable. The formation constant for the first hydrolysis species (log ^*β′₁) determined in the present study is −6.62 ± 0.25. The non-crystalline structure of the plutonium precipitate was observed through X-ray diffraction. The solubility product of Pu(OH)₃ (am), log ^*K′_s,0 is determined to be 15.23 ± 0.50. These results indicate a stronger tendency for the hydrolysis of Pu(III) and higher stability (lower solubility) of Pu(OH)₃ (am) compared to Am(III).