忍者是日本自镰仓时代至江户时代（约1185年到1867年）出现的一种特殊职业身份。忍者们接受忍术（即秘密行动的技术）的训练，主要从事间谍活动。像日本武士的武士道一样，忍者也遵循一套自己引以为荣的专门规范（忍术）。

有不少忍者的传奇故事。忍者盛行于日本战国时代，效力于大名，执行情报收集﹑刺杀等秘密任务。直到德川幕府将其纳入正式编制后，进入最兴盛的年代。不过，虽然如此，但正式的史书上对忍者却只有断断续续的支字词组，也因如此，后人对忍者也就有更多的神秘感。

忍者经常穿着深蓝、深紫色的服饰，因此容易隐匿于星夜之中。一般影视作品都为忍者角色作出纯黑色的乔装，其实是一误解。

A ninja(忍者) or shinobi(忍び) was a covert agent or mercenary in feudal Japan. The functions of the ninja included espionage, sabotage, infiltration, assassination and guerrilla warfare. Their covert methods of waging irregular warfare were deemed "dishonorable" and "beneath" the samurai-caste, who observed strict rules about honor and combat. The shinobi proper, a specially trained group of spies and mercenaries, appeared in the Sengoku or "warring states" period, in the 15th century, but antecedents may have existed in the 14th century, and possibly in the 12th century (Heian or early Kamakura era).