Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula K2[HgI4].

It consists of potassium cations and tetraiodomercurate(II) anions.

It is the active agent in Nessler's reagent, used for detection of ammonia.

Attempted synthesis in concentrated aqueous solution will give the pale orange monohydrate K[Hg(H2O)I3] instead.

At higher concentrations, a brown precipitate derivative of Millon's base (HgO·Hg(NH2)Cl) may form.