An unlockable stage is a stage that is not available at the beginning of the game, (unlike a starter stage), but must be unlocked.

Super Smash Bros.

Stage How to Unlock
Mushroom Kingdom Beat 1-player mode with all 8 starter characters and play every other stage at least once.

Super Smash Bros. Melee

Stage How to Unlock
Eagleland - Fourside Play 100 VS. matches
F-Zero Grand Prix - Big Blue Play 150 VS. matches
Past Stages - Dream Land Clear Target Test with all 25 characters
Past Stages - Kongo Jungle Clear 15-Minute Melee
Past Stages - Yoshi's Island Hit more than 1350ft/400m in Home Run Contest
Kanto Skies - Poké Floats Play 200 VS. matches
Mushroom Kingdom II Unlock the Birdo or Pidget trophys
Planet Zebes - Brinstar Depths Play 50 VS. matches
Special Stages - Final Destination Clear every Event Match
Special Stages - Battlefield Complete All-Star Mode
Superflat World - Flat Zone Complete Classic Mode with Mr. Game & Watch

Super Smash Bros. Brawl

Stage How to Unlock
Donkey Kong 75m Unknown
Big Blue Unknown
Electroplankton Unknown
Flat Zone 2 Unknown
Great Sea Unknown
Green Greens Unknown
Green Greens Unknown
Green Hill Zone Unknown
Jungle Japes Play on any Melee stages in 10 Vs. matches
Luigi's Mansion Play with Luigi in 3 Vs. matches
Mario Bros. Unknown
Pokémon Stadium (original) Play on Pokémon Stadium 2 in 10 Vs. matches
Spear Pillar Unknown

See also