Republicanism is a political ideology centred on citizenship in a state organized as a republic. Historically, it emphasizes the idea of self-rule and ranges from the rule of a representative minority or oligarchy to popular sovereignty. It has had different definitions and interpretations which vary significantly based on historical context and methodological approach.

Republicanism may also refer to the non-ideological scientific approach to politics and governance. As the republican thinker and second president of the United States John Adams stated in the introduction to his famous A Defense of the Constitutions of Government of the United States of America, the "science of politics is the science of social happiness" and a republic is the form of government arrived at when the science of politics is appropriately applied to the creation of a rationally designed government. Rather than being ideological, this approach focuses on applying a scientific methodology to the problems of governance through the rigorous study and application of past experience and experimentation in governance. This is the approach that may best be described to apply to republican thinkers such as Niccolò Machiavelli (as evident in his Discourses on Livy), John Adams, and James Madison.

The word "republic" derives from the Latin noun-phrase res publica (public thing), which referred to the system of government that emerged in the 6th century BCE following the expulsion of the kings from Rome by Lucius Junius Brutus and Collatinus.

This form of government in the Roman state collapsed in the latter part of the 1st century BCE, giving way to what was a monarchy in form, if not in name. Republics recurred subsequently, with, for example, Renaissance Florence or early modern Britain. The concept of a republic became a powerful force in Britain's North American colonies, where it contributed to the American Revolution. In Europe, it gained enormous influence through the French Revolution and through the First French Republic of 1792–1804.