Stamps are collectibles that appear in Super Mario 3D World. They are used to decorate Miiverse posts.
There are a total of 85 of them throughout the game, and are found in every world except World Mushroom and World Flower. The last five stamps, Mario, Luigi, Peach, Toad, and Rosalina's Super Mario 3D World portraits, are collected by completing every single level with the character that the stamp accords to. Five of the courses have buttons with one character's logo on it. Be that character to get the stamp.