Mega Mushrooms are mushrooms that make the user grow to an incredible size and grant invulnerability. They have big yellow caps with red spots, and while all other mushrooms slide along the ground, the Mega Mushroom hops.
In New Super Mario Bros. for the Nintendo DS, Mega Mushrooms cause Mario or Luigi to grow to the size of the entire screen for a limited amount of time, destroying anything in their path including Warp Pipes, enemies, blocks, and even the end-level flagpoles. It also allows the player to kill a boss by walking into it. The megameter is featured at the top of the screen and is filled up by destroying the various objects. When the effect ends, 1-Up Mushroom are awarded based on the portion of the meter that has been filled. If the meter is completely full, the player is awarded five 1-Up Mushrooms. The Mega Mushroom has the same color scheme of the original Super Mushroom from Super Mario Bros..
Mega Mushrooms are also present in Super Mario 3D World but they are a lot less common and can be found on a few levels. Usually, whenever Mega Mushrooms are seen, they are needed to get to a Green Star nearby.
A Mega Mushroom makes an appearance in Mario Kart Wii as an item for racers. Once obtained from an Item Box, the character will grow to a huge size, increasing the speed of their kart. If there are any unfortunate drivers in the way, the racer with the Mega Mushroom item can run them over and squish them, which in turn reduces their speed (and also their size). Also, the same music from New Super Mario Bros when you have a Mega Mushroom plays when you equip this item. Generally, the item is received by competitors in the 4th through 7th places.
Mega Mushrooms also appear in Mario and Sonic at the Olympic Winter Games. They only appear in only two events, Dream Snowball Fight and Dream Curling. In Dream Curling, they make the player's character and the puck bigger. In Dream Snowball Fight, (almost the same thing in Dream Curling) they make the player's character and their snowballs bigger for a short amount of time. If someone gets hit by a big snowball, they pass out for a couple of seconds.
The Mega Mushroom debuts in Mario Party 4. It is an item that allows the player to increase in size. While larger, the player gets to roll two Dice Blocks. This causes the player to bypass all board events, and steal ten coins from other players they pass.