Heiankyo Alien (平安京エイリアン, Heian-kyō Eirian) is a video game created by the University of Tokyo's Theoretical Science Group (TSG) in 1979. The game was originally developed and released as a personal computer game in 1979, and was then published by Denki Onkyō Corporation as an arcade game in January 1980. The game has been ported to several other gaming systems since its original release.

The player controls a Heian period police officer (kebiishi) who must defend the capital city (Heian-kyō) from an alien invasion by digging holes in the ground and filling them back up after an alien falls inside. The player scores points for every alien trapped, and the quicker the hole is filled up after the alien falls in, the higher the amount of points are scored. The aliens increase in number as the levels progress, and they can escape from holes after a certain period of time elapses or if another alien passes above their hole. The player loses if they come in contact with an alien. There is a time limit for each level, and the number of aliens increases drastically when this limit is reached, essentially preventing the player from completing the level.

The player can enter their name on the arcade machine if a high-score is reached. In the original version of the game, a unique cursor is employed where the player selects a letter by having their character walk through the street that displays the desired letter. The game also contains a two-player mode, where the players can either alternate turns or play on the same screen simultaneously.

The arcade version uses a Zilog Z80 microprocessor, and the arcade system board used is a copied version of the one used by Sega's Head On. Since the original system board itself was a copy, Heian-kyō Alien's system board was easily duplicated, and many of the units distributed to arcade centers were "copies of a copy" created by other companies. Few of the original units manufactured by Denki Onkyō remain in place today.