A highly efficient colorimetric and fluorescence turn-off probe for the sensitive and selective detection of the biologically important amino acid, cysteine (Cys), is demonstrated using a newly synthesized water-soluble hyperbranched polymer (HP) containing sulfonic acid groups. The detection mechanism involves two steps: (i) the slight quenching of HP in the presence of Co2+ in advance; and (ii) the gradual quenching of the HPCo2+ complex according to the concentration of Cys due to the absorption screening effect of the formation of the Cys-Co2+ complex, which prevents HP from absorbing excitation energy.