'世上没有免费午餐（简写TANSTAAFL、TINSTAAFL、TNSTAAFL，来自英文There ain't no such thing as a free lunch或There is no such thing as a free lunch，）是经济学谚语，指不付出而获得利益是不可能的。表面上免费的东西，往往会通过其他方式收费，例如隐藏成本及外部成本。

"There ain't no such thing as a free lunch" (alternatively, "There is no such thing as a free lunch" or other variants) is a popular adage communicating the idea that it is impossible to get something for nothing. The acronyms TANSTAAFL, TINSTAAFL, and TNSTAAFL, are also used. Uses of the phrase dating back to the 1930s and 1940s have been found, but the phrase's first appearance is unknown. The "free lunch" in the saying refers to the nineteenth-century practice in American bars of offering a "free lunch" in order to entice drinking customers.