A Hammer Suit is an item that made it's debut in Super Mario Bros. 3 where it was a rare power-up that transforms Mario into Hammer Mario, who could toss Hammers like that of a Hammer Bro. at enemies. At that time, it was a black suit like the Hammer Bros. in that game, but as the Hammer Bros. were redesigned, so was the suit.
This suit generally reappears after a long absence. It appears in a few levels and yet again turns Mario into Hammer Mario.
The Hammer Suit made an appearance in the game New Super Mario Bros. D.I.Y., where the player could make their own New Super Mario Bros. games, therefore, customizing when the Hammer Suit would appear.
Super Mario 3D World
The Hammer Suit appears in Super Mario 3D World. It has a new appearance, it has a shell, green arms, greens shoes, and a green helmet. When Mario or Luigi uses it they have the same appearance as Boomerang Mario. It is a rare item only appearing in Bowser Jr.'s Battleground and Pom Pom's Bombship, it can be bought at Toadette's shop.
In Super Mario Bros. V Hammer Suits are very rare. They can be found in some levels and can be a reward in Brown Toad Houses.