Zeise's salt, potassium trichloro(ethylene)platinate(II) hydrate, is the chemical compound with the formula K[PtCl3(C2H4)]·H2O.

The salt is of historical importance in the area of organometallic chemistry as one of the first examples of a transition metal alkene complex and is named for its discoverer, William Christopher Zeise.

[4][5] In Zeise's salt and related compounds, the alkene rotates about the metal-alkene bond with a modest activation energy.

[6] It was discovered by William Christopher Zeise, a professor at the University of Copenhagen, who prepared this compound in 1830 while investigating the reaction of PtCl4 with boiling ethanol.

[7] Zeise's salt received a great deal of attention during the second half of the 19th century because chemists could not explain its molecular structure.