A Frog Suit is an item found exclusively in Super Mario Bros. 3. The Frog Suit transforms Mario or Luigi into Frog Mario or Frog Luigi. It grants Mario or Luigi the power to jump higher, swim faster, resist water currents better, and hop while walking on the ground. When Frog Mario or Frog Luigi is hit by an enemy, they revert back to Super Mario.

The Frog Suit is a handy item to use in underwater levels. Without one, Mario and Luigi will sink underwater and can only swim diagonally up and to the left, diagonally up and to the right, and straight up. Frog Suits allow them to remain stationary in the water, and swim in any direction. However, Frog Suits don't provide much help on dry land. When out of water, Frog Mario and Frog Luigi move very slowly, as they can only move by hopping (instead of running). On the positive side, Frog Suits do allow the Mario Brothers to jump higher than they can normally.

It is possible to run on dry land with a Frog Suit if the player is carrying a Koopa Shell.

The Frog Suit also appears in quite a good amount of The Adventures of Super Mario Bros. 3 episodes. In "Reptiles in the Rose Garden," Mario uses a Frog Suit to get the White House back on the surface of the Mushroom Sea. In the beginning of "7 Continents for 7 Koopas," Mario & Luigi can be seen crossing a bridge wearing Frog Suits. They were planning to relax in the water with Princess Toadstool and Toad, when King Koopa attacked, thus starting the problems for the Mario Bros. But the most prominent appearance of the Frog Suit in the cartoon was in the episode "The Ugly Mermaid," because Mario, Luigi, the Princess and Toad were all using Frog Suits (and thus creating Frog Princess and Frog Toad), and they kept those suits through the whole episode.