trans-Dichlorodiammineplatinum(II) is the trans isomer of the coordination complex with the formula trans-PtCl2(NH3)2, sometimes called transplatin.

The existence of two isomers of PtCl2(NH3)2 led Alfred Werner to propose square planar molecular geometry.

It slowly hydrolyzes in aqueous solution to give the mixed aquo complex trans-[PtCl(H2O)(NH3)2]Cl.

Oxidative addition of chlorine gives trans-PtCl4(NH3)2. trans-Dichlorodiammineplatinum(II) has had far less impact on medicinal chemistry compared to its cis isomer, cisplatin, which is a major anticancer drug.

Nonetheless, replacement of the ammonia with other ligands has led to highly active drugs that have attracted much attention.