Ancien Régime () refers primarily to the aristocratic, social, and political system established in France under the Valois and Bourbon dynasties (14th century to 18th century). The term is French for "Former Regime," but rendered in English as "Old Rule," "Old Order," or simply "Old (or Ancient) Regime".

As defined by the creators of the term, the Ancien Régime developed out of the French monarchy of the Middle Ages, and was swept away centuries later by the French Revolution of 1789. Europe's other anciens régimes had similar origins, but diverse ends: some eventually became constitutional monarchies, whereas others were torn down by wars and revolutions.

Power in the Ancien Régime relied on three pillars: the monarchy, the clergy, and the aristocracy. Society was divided into three Estates of the realm: the First Estate, Roman Catholic clergy; the Second Estate, the nobility; and the Third Estate, the rest of the population.

More generally, ancien régime (not capitalised, to refer to regimes other than the Ancien Régime in 14th to 18th century France) means any regime which shares the former's defining features. The Ancien Régime retained many aspects of a feudal system that had existed since at least the 8th century, in particular noble and aristocratic privilege, and was supported by the doctrine of the Divine Right of Kings. It differed from that earlier feudal order in that political power had increasingly become concentrated in an absolute monarch.

The term dates from the Age of Enlightenment (first appearing in print in English in 1794) and was originally pejorative in nature. Similar to other sweeping criticisms of the past, such as the consciously disparaging term Dark Ages for what is more commonly known as the Middle Ages, the concept of Ancien Régime is layered onto the past as an expression of disapproval for the way things were done, and carries an implied approval of a "New Order". The term was created by the French Revolutionaries to promote a new cause and discredit the existing order, and was not a neutral historical descriptor of the past.

For some authors, though, the term came to denote a certain nostalgia. Talleyrand famously quipped:

The analogous term "Antiguo Régimen" is often used in Spanish. However, although Spain was strongly affected by the French Revolution and its aftermath, the break was not as sharp as in France.