Which Super Mario game was the first to allow four-player simultaneous play?

The first Super Mario game to allow four-player simultaneous play was New Super Mario Bros. Wii.

Released for Wii in 2009, it was the first main Super Mario platformer to support simultaneous cooperative multiplayer. Up to four players could control Mario, Luigi, Yellow Toad, and Blue Toad together in the same side-scrolling stages.

The feature grew from Shigeru Miyamoto's long-standing interest in cooperative Mario gameplay. Earlier games such as Super Mario Bros. offered two-player alternating turns, while Mario Party used four-player competition in a spin-off format. New Super Mario Bros. Wii put all four characters on-screen at once in the traditional platforming adventure.

Players could jump on one another, carry or throw teammates, ride Yoshi, and rescue fallen players trapped in bubbles. The game also included competitive modes, so four-player sessions could be cooperative or chaotic. Super Mario 3D World later expanded the idea into 3D, but it was not the first game to introduce simultaneous four-player Mario platforming.