A new adsorbent Cu-Perussian blue@Nanodiamond (Cu-PB@DND) for Cs+ removal was prepared and characterized with IR, SEM, X-ray diffraction, particle size analysis, and zeta-potential. The adsorbent consists of a core of aggregated detonation nanodiamond (DND) particles with the surface treated with Cu-PB. Cesium adsorption was studied in two modes; a co-precipitation mode and a batch mode. In the co-precipitation mode, DND, CuCl2, and K4[Fe(CN)6] were added sequentially to a Cs+ solution in diluted artificial seawater. In the batch mode, adsorbent Cu-PB@DND was dispersed into a Cs+ solution with stirring. The distribution coefficient (Kd) of the co-precipitation mode was 8.8 × 107 (mL/g) at Cs+ 6.6 ppm in 0.07% seawater. The Kd value of the batch mode was 1.3 × 106 (mL/g). Precipitation of Cs+-incorporated particles was complete, and post filtration was not necessary. Excess copper and iron ions were completely removed and were not detected in the supernatant. The adsorption data for Cu-PB@DND were analyzed by assuming Langmuir isotherm and a good fit was obtained with a maximum adsorption capacity Qmax of 759 mg/g. The co-precipitation method was also applied to soil-treated wastewater.