Randomness is the chance that an event will occur. It is a critical factor in all video games, including the Super Smash Bros. series. Random events challenge players to think quickly and cause a game to never be played the same way twice.
 Physics of computer-generated random numbers
Computers are unable to generate truly random numbers. They can only create a series of pseudo-random numbers. If a computer were to start out with a certain seed (initial number), one could predict the entire sequence of pseudorandom numbers to follow. It is believed this is how Replays are saved; the system only saves the seed instead of all random events.
 Random events in the Smash Bros. series
- Computer players do things randomly: the higher the computer's level, the more likely it is to do a specified action.
- Random items appear in predetermined places at random times, with preset ratios.
- Tripping in Brawl is a random event that may occur whenever a dash is inputted.
- In Melee and Brawl, a KO by the upper blast line will usually show as a star KO, but there is a chance for a screen KO instead.
 Random character/stage select
The following are random occurrences during character and stage selection.
- In SSB, there was no random feature for selecting characters. However, computer players were selected randomly once entered. Random stage select would choose one of the eight or nine available stages.
- In Melee, once all characters are unlocked, placing one's selection token on either of the two blank corners of the selection grid would move the token onto a random character. CP players would be randomized. Random stage select would select from any of the unlocked stages. Once all stages are unlocked, the player can choose which stages to exclude from the selection list.
- In Brawl, the character selection screen has a designated space for choosing a random character. A "?" appears on the player's selection bracket and no one will know who that player will be until the match begins. (The system will choose and begin loading the character as soon as Random is selected, however.) This random function also selects a random costume for the character, and is how CPUs are entered by default. Randomly choosing a stage behaves the same as Melee, but the player cannot choose to leave out individual custom stages. In addition, custom stages saved on the SD card cannot be randomly chosen.
- When the player selects the Mushroomy Kingdom stage, it will randomly choose between the above ground level and the underground level, though which stage it chooses can be controlled through holding a specific button. The same applies to which Pokémon will appear when Spear Pillar is chosen.
 Random events intrinsic to a character
The following are random occurrences that apply to individual characters.
- The barrels launched from when Diddy Kong's up special Rocketbarrel Boost is broken, have a random trajectory.
- Ivysaur's side special, Razor Leaf, curves in a random direction (up, down, or no curve).
- King Dedede's side special, Waddle Dee Toss, can produce a Waddle Dee (71.4%), a Waddle Doo (20.4%), or a Gordo (8.2%). There's also a small chance that a Capsule, containing a random item, is produced. A Capsule will not appear however, if it is turned off on the Item switch, and only items turned on will appear from the Capsule.
- King Dedede's Final Smash, Waddle Dee Army, produces random minions in the same proportions as the Waddle Dee Toss.
- Kirby can transform into 6 different objects when he uses Stone. This only affects the move aesthetically, and has no effect on gameplay.
- Luigi's side special, Green Missile, has a 12.5% (1/8) chance of misfiring.
- Luigi's Final Smash, Negative Zone, can cause a variety of random, harmful effects.
- Mr. Game & Watch's side special, Judge, has 9 randomly-chosen attacks, each with different strengths and effects. The chance of pulling each number is equal, and Judge will not pull the last two numbers pulled. When the move has not been used in a match, 1 and 2 are considered to be the previous two moves pulled.
- The sausages from Mr. Game & Watch's neutral special Chef, travel in the air randomly.
- Olimar's neutral special, Pikmin Pluck, produces a random Pikmin species. It's influenced by what type of ground Olimar's standing on.
- Peach's forward smash can hit with 3 different weapons: a frying pan, a golf club or a tennis racket. Each weapon has an equal chance of being pulled, and the same weapon will not be pulled twice in a row.
- Peach's down special's Vegetables can have a variety of random faces, with the faces signifying the power of the turnip. The most powerful turnip, the "stichface", has a chance of 1/58 of being pulled. There's also a very small chance of a Beam Sword, Bob-omb, or a Mr. Saturn being pulled instead of a vegetable. And unlike with Capsules being thrown by Waddle Dee Toss, the chance of pulling these items is not affected by their on/off status on the Item switch.
- The trajectory of the projectile launched from Snake's Up Smash, can deviate slightly to the right or to the left.
- Snake has a 16.66% (1/6) chance of planting a C3 instead of a C4 when his down special is used. This only affects the move aesthetically, and has no effect on gameplay.
- There's a rather large chance that Wario will fall from his bike when turning it.
 Random events intrinsic to a stage
The following are random occurrences that occur in specific stage.
 In Super Smash Bros.
- In Hyrule Castle, the tornado hazard appears at random.
- When entering a pipe in Mushroom Kingdom (SSB), the destination pipe is chosen at random.
- On Saffron City, the Pokémon that appears from the door to the Silph Co. building is chosen at random.
 In Super Smash Bros. Melee
- The layout of the track in Big Blue is randomly generated.
- The rate at which the acid in Brinstar rises and falls is randomly chosen from preexisting patterns.
- The stage in Brinstar Depths turns in a random direction when hit by Kraid.
- Arwings and Wolfens appear at random on Corneria, as well as have randomly chosen flight paths, and attack at random.
- The stage hazards on Flat Zone appear at random.
- The two lower platforms on Fountain of Dreams slowly move up and down in a random pattern.
- On Green Greens, whether a falling block is a normal block or bomb block is chosen randomly.
- Which direction the stage moves in on Icicle Mountain, as well as the speed it moves, is random.
- On Mushroom Kingdom, ? blocks instead of normal blocks spawn randomly.
- When and which cars drive by on Onett is random, though a sound and visual warning is given before they drive by.
- On Pokémon Stadium, which stage transformation occurs is chosen at random. However, it'll cycle through all four transformations before repeating, and a long warning is given before the stage transforms.
- Switches and Banzai Bill will appear on Princess Peach's Castle at random.
- Like on Corneria, Arwings and Wolfens appear at random on Venom.
- On Yoshi's Story, Shy Guys appear at random in packs up to six, which carry food if items and food are turned on. These Shy Guys can be attacked, and will drop their food if carrying it.
 In Super Smash Bros. Brawl
All returning Melee stages retain their random elements from Melee.
- Donkey Kong wakes up at random on 75 m, and the amount of time he is out is random.
- King Bulblin appears at random on Bridge of Eldin, and will drop a bomb on the center of the stage at random (though he'll always drop a bomb during his first appearance). Bulblins straggling behind King Bulblin, which can be attacked, will also appear at random.
- The order of the locales in which Delfino Plaza stops at is random. When the stage rises up to move from a stopped at location, its platform arrangement is chosen randomly from preexisting patterns.
- Platforms appear at random during the beginning portion of Flat Zone 2, as well as the treasure diver to collect items. Which game the stage transforms to is also chosen at random.
- How often the stage flips on Frigate Orpheon is chosen randomly from preexisting patterns.
- Where the checkpoint lampposts appear on Green Hill Zone is random. Different portions of the stage will also crumble away at random, if players do not destroy these stage portions themselves.
- On Halberd, the Combo Cannon on the bridge portion of the stage will attack players at random, with a randomly chosen weapon. When the stage rises up from the bridge, whether or not the camera is properly zoomed in until the next bridge stop, also occurs randomly.
- The stage on Lylat Cruise slowly tilts at random.
- When and which stage hazards on Mario Bros. occur is random.
- When and where the Ultimate Chimera spawns on New Pork City occurs randomly.
- Stage hazards on Norfair occur at random. Similar to Brinstar, the pattern of the lava's rising and dropping is chosen randomly from preexisting patterns.
- Which transformation on Pictochat occurs is random, though like Pokémon Stadium, it'll cycle through all possible transformations before repeating.
- Stage events on Pirate Ship occur at random.
- Like with Pokémon Stadium, which stage transformation occurs on Pokémon Stadium 2 is chosen at random, with the stage cycling through the four transformations before repeating, and gives players a long warning before transforming.
- Which location is stopped at on Port Town Aero Dive is chosen randomly. Cars will also drive by some of these stops at random.
- When and which Metal Gear appears in the background on Shadow Moses Island is chosen randomly. This has no impact on gameplay however.
- The starting position of the floating platform on Smashville is random, as well as the occurrence of the balloon.
- The background Pokémon in Spear Pillar attacks at random, with randomly chosen attacks.
- Food from Ice Climber spawns randomly on Summit.
- On WarioWare, Inc., the stage transforms into a randomly chosen microgame. When a player wins one of these microgames, they are randomly rewarded with the effects of a Starman, Super Mushroom, Food, or nothing.
- On Yoshi's Island, the support ghost appears in predetermined places at random. Like Yoshi's Story in Melee, Shy Guys carrying food (if items and food are turned on) will appear randomly in packs up to four, which can be attacked to cause them to drop their food. Which season the stage starts on is also chosen randomly, though this has no effect on gameplay.
 Random events intrinsic to an item
The following is random behavior of specific items.
 In all Super Smash Bros. games
- Which items appear from container items is chosen randomly from what items turned on. There's also a random chance of containers not containing any items and instead being explosive (with the exception of Sandbag).
- What Pokémon a Poké Ball spawns is random, with each Pokémon having a different chance of appearing. If the Pokémon targets a specific player, which opponent is targeted is also chosen at random.
 In Super Smash Bros. Melee
- When food spawns, the specific food that appears is random.
- The Hammer head on a Hammer will drop off at random when the Hammer is picked up.
 In Super Smash Bros. Brawl
All returning items from Melee retain any random elements they had in Melee.
- Like with Poké Balls, which character an Assist Trophy spawns is chosen randomly. Which opponent the Assist Trophy character targets (if any) is also random.
- Occasionally, Electrode will fail to explode. However, after a short time, Electrode will explode anyway, and will explode when thrown.
- Golden Hammers will occasionally spawn as a Golden Squeaky Hammer at random, which while indistinguishable from appearance, produces hitboxes that are completely harmless, and produce a "squeaky" noise when it hits someone.
- Instead of turning all opponents small, Lightning may randomly turn the player who grabbed it small, or turn all opponents giant.
- Instead of slowing all opponents down, a Timer may randomly slow down the player who grabbed it, or slow down all players.
- Similarly to Electrode, Smart Bombs occasionally fail to explode when thrown or hit. They will eventually explode if left uninterrupted, or will explode if they come in contact with a hitbox that deals fire damage.
 Randomness in competitive play
While some randomness that can be controlled and/or has little impact on gameplay is considered acceptable, if undesirable, in competitive play, excessive, uncontrollable randomness is usually considered degenerate to competitive play. The reasoning being that, if randomness has too much of an impact on gameplay, the winner of the match could simply win by being luckier with random events, rather than outplaying their opponent. Thus to prevent people from winning by being the luckier player, and to ensure the more skilled player wins, steps are taken to limit the effect of randomness in competitive play. These steps include turning off all items, using stage striking instead of random select to choose the stage for the first game in a tournament set, and banning stages whose random events have too large of an impact on gameplay within the stage (such as WarioWare).