A novel aminophenylthiomethyl chitosan (APTMC) derivative was synthesized by reacting chitosan with 2-amino-benzenethiol, and used to selectively separate precious metals such as gold(III), palladium(II) and platinum(IV) from base metal solutions. Firstly, the adsorption selectivity of APTMC for metal ions in a hydrochloric acid solution was investigated. The adsorption of metal ions on APTMC was dependent on the hydrochloric acid concentration. Gold(III), palladium(II) and platinum(IV) are selectively adsorbed at a lower concentration among hydrochloric acid concentrations, and then their adsorption percentages decrease with increasing hydrochloric acid concentration. This indicates that gold(III), palladium(II) and platinum(IV) can be selectively separated from base metal solutions with APTMC by adjusting the hydrochloric acid concentration. The adsorption equilibria of precious metals such as palladium(II) and platinum(IV) were then quantitatively examined. The adsorption selectivity of palladium(II) from a mixed solution of palladium(II)/iron(III) was demonstrated with APTMC, even in the presence of about a 500-fold excess of iron(III). Finally, the adsorption-desorption cycle of palladium(II) and platinum(IV) adsorbed on APTMC was carried out using an aqueous thiourea solution containing hydrochloric acid. There was no signicant decrease in the adsorption capacity on APTMC, indicating that it could be reused 5 times for the adsorption of palladium(II) and platinum(IV).