Thiosemicarbazidedichloromercury(II) was synthesized in an ethanol–water solution by keeping the molar ratio of HgCl₂ and thiosemicarbazide (tsc) at 1:1. The crystal is monoclinic: a = 8.966(11), b = 6.817(9), c = 12.092(14) Å, β = 100.3(1)°, and space group P2₁/c with four molecules per unit cell. The tsc is a bidentate ligand in this complex with Hg—S = 2.417(6) Å, Hg—N = 2.51(2) Å. The mercury atom is trigonal pyramidally four-coordinated, with two Hg—Cl distances of 2.366(6) and 2.770(5) Å. When the tsc is coordinated to the mercury(II) ions, the chemical shift of the ¹H atoms on the NH₂ and NH groups shift to a lower field.