Flat Zone (フラットゾーン, Flat Zone) is an unlockable stage in Super Smash Bros. Melee, and the home stage of Mr. Game & Watch. Flat Zone is unlocked when the player beats Classic Mode with Mr. Game & Watch. In All-Star mode, Flat Zone appears as the last stage where the player faces 25 lighter Mr. Game and Watches. Mr. Game & Watch's Target Test also takes place on a modified version of this stage. Also, two event modes in Super Smash Bros. Melee also appear fighting on this stage.
While the stage itself has never been brought back as a standalone stage, it appears as one of the transformations in Flat Zone X, a mixture between Flat Zone and Flat Zone 2 (from Brawl) in Super Smash Bros. for Wii U.
Flat Zone is a small stage. In the middle of the stage there are numerous lines that characters can jump on. The lines will fade out and reappear while the characters battle. On the far right of the screen, characters can stand on the roof of the house. Characters will appears as 2-D images on the screen, and make a beeping noise whenever they walk. When the game is paused, rather than zoom in on one character, the camera zooms out to show the entire Game & Watch device being played on. The layout is heavily based on the Game & Watch game Helmet.
From 1980 until 1991, Nintendo has produced handheld devices called Game & Watch. There is one game for each model of a Game & Watch. The screens used on an LCD screen, where they used on an ordinary calculator, so the character and other moving objects could only move to pre-determined spot and everything on the screen appeared flat. In this stage, the characters and the elements in the screen are all flat and 2-D. The overall design of this stage is very similar to the Game & Watch game Helmet. The goal of Helmet is to try to get from the building on the left to the building on the right without get hit by falling tools. The majority of Game & Watch games make a beep sound whenever something happens on the screen.
This stage is also partly based on the Game & Watch game Manhole. The goal is for the player to fill in spots of sewer holes so that pedestrians don't fall in. If a hole wasn't filled the walking pedestrians would fall in the sewer. In this stage there are platforms that appear in the middle of the stage. The character used in Manhole is seen filling in the spaces between the platforms.
Another Game & Watch game is Oil Panic which goal is for the player to collect dripping oil in a bucket and dump it to another guy holding a bucket below. If the player fails and misses the guy holding the bucket oil, the item is dumped all over a customer at the gas station. The Oil Panic character in this stage holding an oil drum and spilling on the ground is a possible reference seeing that the player can spill oil on a customer. When a player walks on this stage, a beep sound can be heard. 
- Flat Zone 2
This stage of Super Smash Bros. Melee takes its motif from the Game & Watch series. It incorporates settings from Oil Panic, Helmet, and Manhole, and it's sure to send a wave of nostalgia crashing over old-school gamers. If you want to truly reproduce the Game & Watch experience, try playing this unique level in Fixed-Camera Mode!.
- Game & Watch
Flat Zone is listed as a banned stage in tournaments in both the singles and doubles sets; this is largely due to the small size, which allows for very simple KOs because of closer blast-lines. Along with this, the falling tools can cause one-hit KOs, and is difficult to avoid due to their falling speed.
- Even though all characters become 2D when playing on this stage, all items remain 3D.
- When characters are Screen KO'd on Flat Zone, they will hit the screen in their standard tumbling animation instead of their animations for other stages, with the exception of Mr. Game & Watch, who has only one tumble animation.
- In Training Mode, when the camera is at normal, it will zoom closely on the character, and on zoom, it will show the whole stage.
- When playing as Master Hand (through either the Master Hand Glitch or hacking), the game will normally crash, as Master Hand has no 2D appearance. Oddly, an Invisible Melee can prevent this.
- This stage, along with Onett, are the only stages in Melee to not have any lower blast lines.
- This stage is the smallest stage in Melee.
- The speed on this stage seems to be slightly faster than that of a regular battle.