In this work, dialdehyde cellulose nanocrystals (DCNCs) were prepared via oxidizing cellulose nanocrystals (CNCs) with sodium periodate. The changes in the morphology of CNCs were evaluated by means of Fourier transform infrared spectrophotometry (FTIR), X-ray diffraction (XRD), and atomic force microscopy (AFM). The product (DCNC) was used as an adsorbent for creatinine. The results showed that the adsorption capacity of DCNCs increased with the increase in its aldehyde content. With a decrease in the size of DCNCs, the adsorption of creatinine on DCNCs became faster. The results revealed that the adsorption isotherm of DCNCs agreed with the Freundlich adsorption model and the adsorption process followed a second-order kinetic model. Overall, the maximum adsorption of creatinine on DCNCs (with 4.41 mmol/g aldehyde content) was 1.50 mg/g, which was obtained under the adsorption conditions of 100 mg/L initial creatinine concentration, 8 h treatment time, and pH 7. The results suggest that the DCNCs may have potential to be used as an adsorbent for creatinine to treat chronic renal failure.