Zeise's salt, potassium trichloro(ethene)platinate(II), is the chemical compound with the formula K[PtCl₃(C₂H₄)]·H₂O. The anion of this air-stable, yellow, coordination complex contains an η²-ethylene ligand. The anion features a platinum atom with a square planar geometry. The salt is of historical importance in the area of organometallic chemistry as one of the first examples of a transition metal alkene complex.