Which item evolves Poliwhirl into Politoed?

Poliwhirl evolves into Politoed when traded while holding a King's Rock. This trade-based evolution was introduced in Generation II, alongside Politoed as a new branch in the Poliwag family.

The King's Rock is a held item shaped like a crown. In addition to triggering Poliwhirl's evolution, it can make certain moves cause the opponent to flinch. The item is consumed when the trade evolution occurs in games that use this mechanic.

A common mix-up is confusing the King's Rock with the Dragon Scale or Metal Coat. The Dragon Scale evolves Seadra into Kingdra, while the Metal Coat evolves Onix into Steelix or Scyther into Scizor. The King's Rock also evolves Slowpoke into Slowking, making it one of the few evolution items shared by two species.

In Pokémon GO, the same item is used for Politoed, but the process is different: Poliwhirl requires Candy and a King's Rock rather than a link trade. The core association remains the same across the series: King's Rock is Politoed's signature evolution item.