A peptidyl chemosensor (1) showed sensitive colorimetric and fluorescent responses to Hg²⁺ and Ag⁺ among 16 metal ions in aqueous solutions. 1 detected sensitively Hg²⁺ and Ag⁺ by color change with a red shift of the maximum absorbance band at 452 nm; a red color for Hg²⁺ and an orange color for Ag⁺. Furthermore, 1 showed a highly selective colorimetric response to Hg²⁺ among 16 metal ions in aqueous solutions containing NaCl. The selective colorimetric detection of Hg²⁺ was not interfered by other metal ions. About 2 equiv. of Hg²⁺ was enough for the saturation of color and fluorescent change of 1 in aqueous solutions. The dissociation constant for Hg²⁺ was 2.4 × 10⁻¹⁰ M² (R² = 0.993) and the detection limit for Hg²⁺ was 25.6 nM (R² = 0.994). The binding mode of 1 with Hg²⁺ ions was proposed on the basis of the results of pH titration experiment, NMR titration experiment, and mass spectrometry.