共和制(英語:Republic),是一種國體。與神權及君主制政權的正當性、取得方式和政體形式不同,該政體下國家的最高執政者不是君主,而是依法律選出的最高執政者[1][2]。施行共和制的国家通常称作「共和国」[3][4],在政治思想上,共和主義指以共和制治國的一種意識型態。在現代國家興起後,共和主義在世界各國的實踐及理論發展上各異,但大多數有幾項特徵:人民不是統治者的所有物或附屬品,並且大多擁有一個有效憲法確保人民的基本權利不受政府侵犯;政府權力是公有物,國家的治理是所有公民的共同事業。
we may say with truth and meaning, that governments are more or less republican as they have more or less of the element of popular election and control in their composition; and believing, as I do, that the mass of the citizens is the safest depository of their own rights, and especially, that the evils flowing from the duperies of the people, are less injurious than those from the egoism of their agents, I am a friend to that composition of government which has in it the most of this ingredient.[31]