Alien Enemies (エイリアン eirian) are a type of Enemy Unit. They are introduced in Chapter 1 of Into The Future. They appear in Into the Future, Cats of the Cosmos, Stories of Legend and some events. They are rather strong, generally don't give much money, and have a peculiarity: Their Health and Damage are multiplied by 700%, making them EXTREMELY strong, unless a player has all Superior anti-Alien treasures. Each 100% Treasure reduces this multiplier by 100% (66% reduces it by 66%, 33% by 33%, and so on).

Alien enemies introduced in Cats of the Cosmos are "Starred", which means they have a 1600% strength magnification by default and are unaffected by Into the Future's anti-Alien Treasures. However, Cats of the Cosmos has many Treasures that reduce the power of Starred Aliens. Like the Into the Future Treasures, each 100% Treasure reduces their power by 100%. The stared aliens also have a special ability most comonily having a chance to teleport cats.