Alien Soldier (エイリアンソルジャー) is a side-scrolling run and gun video game developed by Treasure for the Sega Mega Drive. The game was released in Japan and Europe, but not physically in the US (one of the few to be released in this pattern), but it can be rather expensive due to its rarity in either region. The game was playable in America on the Sega Channel cable service and has been reissued for PlayStation 2 as part of the Sega Ages Treasure Box disc. The Sega Ages version, whose disc also includes Treasure's Gunstar Heroes and Dynamite Headdy, exceeds the Mega Drive's intrinsic sprite display limit, which eliminates slowdowns or missing graphics during busy scenes (e.g. boss fights with many explosions). Also, the player can select either the sprite-based pixelated graphics of the original game or a new mode that uses higher-quality sprites re-created by Treasure". The game was re-released again on the Nintendo Wii's Virtual Console download service in Japan on October 9, 2007, in Europe on November 2, 2007, and North America debut on November 5, 2007.

The game is listed in Guinness World Records Gamers Edition 2010 under the category "Most boss battles in a run and gun game".

The message that appears on the Press Start screen "VISUALSHOCK! SPEEDSHOCK! SOUNDSHOCK! NOW IS TIME TO THE 68000 HEART ON FIRE!" is a reference to the Motorola 68000, the CPU of the Mega Drive.

Alien Soldier is unique among side-scrolling shooters in that, instead of long levels with several minor enemies before reaching the boss, the levels are notably short and easy before reaching a boss. This results in the game being mostly boss fights. The game has 25 levels and 31 bosses in total, and two difficulty levels, Supereasy and Superhard. The difficulty of the two levels is largely attributable to the lack of continues (and password-based "saving") available in the Superhard game, which is enabled by default.

The top of the screen is dominated by a status bar which gives information about the player current and maximum health, the current and maximum energy of the selected weapon and the current and maximum health of the boss of the stage.

The player can alter the status bar at the beginning of the game to show each details either in numerical format, a bar format, a series of "????", or a combination of the three. If the player wants more challenge, it is possible to make all of the details to be "????", thereby denying any visual information of the health of the player, weapon energy and the health of the boss. Similarly, if the player wants to be more careful, the player may change the status bar to give numerical details so as to know exactly how much health and energy remains.

Another feature is that if the player were to be hit by an enemy or projectile that would have been fatal, the player's current health will always be reduced to 1 first. The player will only die if he gets hit thereafter, reducing health from 1 to 0. This, in a way, gives the player a second chance to recover and continue with the game.