Mercury(II) is an unphysiological soft ion with high binding affinity for thiolate ligands. Its toxicity lies in the interactions with low molecular weight thiols including glutathione and cysteine-containing proteins that disrupt the thiol balance and alter vital functions. However, mercury can also be detoxified via interactions with Hg(II)-responsive regulatory proteins such as MerR, which coordinates Hg(II) with three cysteine residues in a trigonal planar fashion (HgS3 coordination). The model cyclodecapeptide P3C, c(GCTCSGCSRP) was designed to promote Hg(II) chelation in a HgS3 coordination environment through the parallel orientation of three cysteine side chains. The binding motif is derived from the dicysteine P2C cyclodecapeptide validated previously as a model for d10 metal transporters containing the binding sequence CxxC. The formation of the mononuclear HgP3C complex with a HgS3 coordination is demonstrated using electrospray ionization mass spectrometry, UV absorption, and 199Hg NMR. Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the Hg(II) coordination environment is T-shaped with two short Hg–S distances at 2.45 Å and one longer distance at 2.60 Å. The solution structure of the HgP3C complex was refined based on 1H–1H NMR constraints and EXAFS results. The cyclic peptide scaffold has a rectangular shape with the three binding cysteine side chains pointing toward Hg(II). The HgP3CH complex has a pKa of 4.3, indicating that the HgS3 coordination mode is stable over a large range of pH. This low pKa value suggests that the preorientation of the three cysteine groups is particularly well-achieved for Hg(II) trithiolate coordination in P3C.