偷窃（或盗窃、偷盗），是基于自己或第三人的无正当权利占有（包括管领、支配、处分等），而擅自取走他人财产的行为，是违反社会道德规范，法律上，偷窃也是刑法规定的刑事犯罪行为。做出这个行为的人被称为“小偷”、“窃贼”等。

偷窃犯罪的构成要件通常是意图自己或第三人的不法所有（管领、支配、处分等），且有认知其为他人财产的能力，而故意未经同意地取走他人财产，为侵犯财产法益的犯罪。至于无不法占有的意图，而未经同意使用他人物品，且物主的支配权没有受到超出一般人所容许的损害，则属于学理上之"使用窃盗"，就不在规范范围了。

In common usage, theft is the taking of another person's property without that person's permission or consent with the intent to deprive the rightful owner of it. The word is also used as an informal shorthand term for some crimes against property, such as burglary, embezzlement, larceny, looting, robbery, shoplifting, library theft, and fraud (i.e., obtaining money under false pretenses). In some jurisdictions, theft is considered to be synonymous with larceny; in others, theft has replaced larceny. Someone who carries out an act of or makes a career of theft is known as a thief. The act of theft is known by terms such as stealing, thieving, and filching.