Japan is facing a challenge of wide-spread radioactive contamination by isotopes of cesium (Cs). Here we demonstrate that titanium oxide (TiO₂) can be an efficient material to immobilize high concentrations of Cs ions in a durable solid-state framework. TiO₂ was dissolved in a Cs-containing melt of molybdenum trioxide (MoO₃) at 950^oC. The melt was then electrolyzed at −1200mV (vs. a Pt reference electrode) to obtain a single-crystalline titanate, Cs_0.169TiO₂ (titanium-oxide immobilizer) , which contained 1g cm⁻³ of Cs. Moreover, the titanium-oxide immobilizer exhibited a 170 times lower Cs leaching rate than did Cs-containing borosilicate glass. The titanium-oxide immobilizer will contribute to the Cs decontamination because of its high Cs content and low Cs leaching.