Shadow of the Ninja (闇の仕事人 KAGE, Yami no Shigotonin KAGE, "Dark Professionals: SHADOW") is a futuristic ninja-themed side-scrolling action game by Natsume for the Nintendo Entertainment System originally released in Japan in 1990 and in North America and Europe in 1991. The PAL version was published by Taito under the title Blue Shadow.

Natsume began development of a Game Boy version of Shadow of the Ninja following the original NES release. However, the publishing rights of the game was were picked up by Tecmo and the title was subsequently revised as a Ninja Gaiden spinoff titled Ninja Gaiden Shadow, also developed by the Nagoya division of Natsume.

Shadow of the Ninja can be played by up to two players simultaneously, with one player controlling Hayate (the male ninja) and the other as Kaede (the female ninja). In either mode, the player can decide which character they want to control before the game actually begins. There are no actual performance differences between either character, however they look almost completely different in-game (and in other versions' manga-style covers) than as shown in the US cover art: Hayate is blonde and Kaede is green-haired, wearing a miniskirt and wielding a sword and a kunai.

The player character can run, crouch, climb ladders, attack, and jump like in most side-scrolling action games as well as hang onto an overhang and move under it. If the player has more than half of their vitality remaining, they can perform a special attack by holding a button for an extended period, summoning a thunderstorm that will damage all on-screen enemies, but at the cost of roughly half of their maximum vitality.

The player's default weapon is a katana, which can be traded for a kusarigama and vice-versa. The kusarigama has a longer range than the katana and can be swung upwards diagonally and horizontally, but can only damage enemies from a specific distance and does not work as well at close range like the katana. If the player picks up a weapon they already have, its attack power will be strengthen by an increment of one level (with up to three attack levels). However, if the player takes too much damage, their weapon's strength will be reduced back to its previous level. There is also the possibility to use both weapons, if the player starts with the kusarigama and collects secret powerups hidden in some levels. The player can also obtain shurikens and hand grenades as well, both which can only be used as long the player's supply lasts before the player reverts back to using the katana or kusarigama. All four weapons are obtained by destroying item boxes scattered throughout each stage, along with vitality potions.

The game consists of five levels with 16 stages, with the first four levels being divided into three stages and the final level into two. The player will fight numerous types of enemy characters throughout each level, including bosses and sub-bosses. Once a boss has been defeated it will explode, this explosion will heal players the closer they are from it, since actually the same amount of health from one level will be carried to the next one until either recovered with potions or by dying. The game ends if the player loses all of their vitality and only five chances are provided to continue. If two players are playing the game, their continues will be shared.