What Wii U app let a Nintendo 3DS be used as a controller for Smash for Wii U?

The Wii U app that let a Nintendo 3DS serve as a controller for Smash for Wii U was Smash Controller.

Nintendo released Smash Controller through the Nintendo eShop on June 14, 2015. The application allowed a Nintendo 3DS system to provide controls for Super Smash Bros. for Wii U, giving players another local multiplayer option beyond the Wii U GamePad, Wii Remotes, Classic Controllers, Wii U Pro Controllers, and GameCube controllers with an adapter.

The feature is often confused with Nintendo Switch Online, a much later smartphone service used for voice chat and related functions. It is also separate from Miiverse, Nintendo’s discontinued social network, and from Smash World, a name associated with sharing content in later Smash contexts.

Players could also use the regular Nintendo 3DS version of Super Smash Bros. as a controller for the Wii U game. Smash Controller was the dedicated app for people who wanted that function without using the 3DS game cartridge or software.