Zillion, known as Akai Koudan Zillion (赤い光弾 ジリオン, lit. Red Photon Zillion) in Japan, is a space adventure platform video game designed for the Sega Master System as a companion for the Zillion anime in 1987. The game is a free-scrolling platform/adventure in the vein of Metroid. It also has objects to be inspected and the necessity of enable things by accessing codes in vein of Impossible Mission. A sequel to the game, Zillion II: The Triformation was also released.

Zillion prominently features Sega's egg-shaped mascot, Opa-opa, who stars in the Fantasy Zone games, also available for the Sega Master System.

The player starts out in the role of JJ, just outside the Mothership on the surface of the planet. After reaching the underground base shortly after beginning, the player spends the rest of the game underground, only coming back up to replenish the character's health or to leave after completing the mission.

Throughout the base, are capsules containing key codes and power-up items. Some of the obstacles of the missions include tripwires, enemy guards, laser turrets and force fields. The game style includes entering and exiting room to room in the base and unlocking each room with computer ID cards and inputting the correct four-digit code, found by investigating capsules in the room. Aside from unlocking rooms, the player has also the option to make specific actions, such as turning off barriers, deactivating traps, or even committing suicide, among others. One of the codes is needed to initiate the Noza base self-destruct sequence. The game is well known for a variety of "special messages" that can be received if the player inputs certain codes in certain rooms.

The player carries a gun, which was used as a design model for the Sega Master System Light Phaser. As the game progresses, the gun will become stronger, allowing the player to break progressively stronger capsules containing codes for the computers or powerups. Once they are rescued, the player can also play as Apple or Champ, each one with their own differences; Apple is fast, but weaker and Champ is slower, but stronger. Like JJ, they can also find upgrades for their gun power, speed, jumping ability and health.

Glitches

A well known glitch in the game allows the player to become invincible by dying while going down in a room-to-room elevator. However, using this glitch makes the status menu inaccessible, so the player can only switch characters when a new teammate is found.