埃马纽埃尔-约瑟夫·西哀士（法语：Emmanuel-Joseph Sieyès，1748年3月3日－1836年6月20日），法国天主教会神父，法国大革命、法国执政府和法兰西第一帝国的主要理论家之一，法国督政府督政官、法国执政府执政官。西哀士的《什幺是第三等级》成为大革命的宣言并促使三级会议成立国民议会。1799年，西哀士煽动雾月政变，将拿破仑·波拿巴带上权力顶峰。

Emmanuel Joseph Sieyès (3 May 1748 – 20 June 1836), most commonly known as the Abbé Sieyès (French: [sjejɛs]), was a French Roman Catholic abbé, clergyman and political writer. He was one of the chief of political theorists of the French Revolution, and also played a prominent role in the French Consulate and First French Empire. His 1789 pamphlet What is the Third Estate? became the de facto manifesto of the Revolution, helping to transform the Estates-General into the National Assembly in June 1789. In 1799, he was among the instigators of the coup d'état of 18 Brumaire (9 November), which brought Napoleon Bonaparte to power. He also coined the term "sociologie" in an unpublished manuscript, and made significant theoretical contributions to the nascent social sciences.