The Guardian Legend, known in Japan as Guardic Gaiden (ガーディック外伝, Gādikku Gaiden), is a 1988 hybrid action-adventure/shoot 'em up video game developed by Compile for the Nintendo Entertainment System (NES). It is the sequel to the 1986 MSX game Guardic, and was published and released in Japan by Irem in 1988, in North America by Brøderbund in 1989, and in Europe by Nintendo in 1990. In the game, the player controls a lone protagonist, the Guardian, who is on a quest to destroy a large alien-infested world named Naju before it reaches the planet Earth. The player must deactivate ten safety devices scattered throughout Naju, thus activating the alien world's self-destruct sequence. The player explores Naju in a non-linear fashion and can acquire different weapons during the course of the game.

The Guardian Legend received mixed reviews from magazines such as Electronic Gaming Monthly and Nintendo Power. While it has been praised for its impressive graphics, memorable sound, and responsive controls, it has been criticized for its repetitive gameplay and complicated password system. The game received little fanfare upon its release, but it has since been considered a classic example of a multiple-genre game that set a standard for others such as the title Sigma Star Saga and incorporated gameplay elements from other titles such as The Legend of Zelda, Metroid, 1942, and Blaster Master.

Gameplay varies depending on the player's location within Naju. The player controls the Guardian in humanoid form when exploring the surface of Naju (the Labyrinth) and in spaceship form when investigating Naju's interior (the Dungeon). The Guardian has a life meter that decreases after sustaining damage from enemies; it can be replenished by collecting various items. If the life meter runs out, the Guardian explodes, and the game ends. The player can use a primary rapid-fire weapon with unlimited ammunition as well as various powerful secondary weapons that consume "power chips" with each use. Power chips are also used as currency to purchase upgrades for the Guardian in a handful of shops throughout Naju. Found within the Labyrinth or obtained after defeating a boss, these upgrades include primary weapon improvements, new or upgraded secondary weapons, and round, brightly colored creatures called Landers.

Blue and Red Landers, recurring characters in many Compile games, increase the player's maximum life and power chip capacities, respectively. Blue Landers play multiple roles in The Guardian Legend. Some of them are not items but non-player characters that dispense advice to the player or exchange upgrades for power chips; others provide a password that allows the player to resume the game at a later time with their progress retained. These Blue Landers also serve as checkpoints; players can restart their game in these designated rooms after being defeated provided the system has not been turned off.

In the action-adventure portion of the game known as the "Labyrinth", the player explores the surface of Naju in humanoid form in a top-down perspective. The player must navigate the Labyrinth and find and infiltrate the corridors and ultimately activate Naju's ten safety devices. The Labyrinth consists of screen-wide passages and rooms individually plotted as X–Y coordinates. A map that details these coordinates in a grid-like form can be viewed on the pause subscreen. While the player can generally walk from one screen to the next, some screens are separated by portals called "warp panels". Warp panels bear a symbol indicative of their surrounding area, and the player can only access these warp panels with keys that match these symbols. Some warp panels lead to rooms containing various clues and story elements while others are gateways to shops, password rooms, and corridors. Keys allow players to access different portions of the Labyrinth, which they can then explore in a non-linear fashion.

In the shoot 'em up portion of the game known as the "Dungeon", the player battles through Naju's interior in spaceship form. The Dungeon consists of a series of enemy-filled corridors which are found during exploration of the Labyrinth. The player's objective in the Dungeon is to progress through each corridor and defeat the boss at the end. Upon completion, the player destroys the corridor and is returned to the Labyrinth, where a power-up (and sometimes a warp panel key) is collected as a reward. While some corridors can be accessed freely, others can only be entered by performing a particular action in the corridor room. Some rooms in the Labyrinth contain clues that indicate how to unseal these corridors. Ten of the corridors in the game serve as the safety devices which must be activated to win the game.