君士坦丁献土（拉丁语：Donatio Constantini），是一份伪造的罗马皇帝法令，内容是罗马皇帝君士坦丁一世将罗马一带的土地赠送给教宗。一般认为这份文档被伪造于8世纪至9世纪。在一些教宗致信中，发现当时的教会是真心相信该文档的真实性。如1054年的教宗利奥九世致君士坦丁牧首米恰尔一世·色路拉里乌的信件中，已经将之视为真实文档引用了。而这一文档的最初使用年代在1913年版《天主教百科全书》第五卷《君士坦丁的捐赠》条目中也提到：

第一位在公文中引用并信赖这份文档的教宗是利奥第九；1054年的一封致君士坦丁堡牧首米恰尔一世·色路拉里乌的信件中他提到那"Donatio"显示了教廷是创建在世俗及属灵二种意涵上的王国.

The Donation of Constantine (Latin:Donatio Constantini) is a forged Roman imperial decree by which the emperor Constantine the Great supposedly transferred authority over Rome and the western part of the Roman Empire to the Pope. Composed probably in the 8th century, it was used, especially in the 13th century, in support of claims of political authority by the papacy. Lorenzo Valla, an Italian Catholic priest and Renaissance humanist, is credited with first exposing the forgery with solid philological arguments in 1439–1440, although the document's authenticity had been repeatedly contested since 1001.