A co-precipitation separation method is described for the determination of cadmium in sediment by isotope dilution inductively coupled plasma mass spectrometry. A sediment sample is digested with HNO₃, HClO₄ and HF, and then ammonia solution is added. Zirconium and Sn, which are sources of spectral interference on Cd isotopes in ICP-MS measurements, are co-precipitated with hydrolysis compounds of Al and Fe (major constituents in the digested solution). However, cadmium is masked with ammonia and still remains in the supernatant. The efficiency of the removal of Zr and Sn in the co-precipitation separation are both around 99.5%, and the recovery of Cd is >80%. The measured n(¹¹¹Cd)/n(¹¹²Cd) ratios in the unspiked digested solution of sediment reference materials (PACS-1 and NIES No. 2) after the co-precipitation and separation are in good agreement with those for a standard Cd solution, and the procedural blank of the co-precipitation separation is extremely low (below the detection limit of ICP-MS). The analytical results obtained for Cd in PACS-1 and NIES No. 2 were 2.396 ± 0.033 µg g⁻¹ (value ± expanded uncertainty) and 0.819 ± 0.011 µg g⁻¹, respectively, which are in good agreement with the certified values (PACS-1: 2.38 ± 0.20 µg g⁻¹, and NIES No. 2: 0.82 ± 0.06 µg g⁻¹). This pretreatment method is simple and reliable for the determination of Cd in sediments by ID-ICP-MS.