Cordon sanitaire (French pronunciation: [kɔʁdɔ̃ sanitɛʁ]) — or quarantine line — is a French phrase that, literally translated, means "sanitary cordon". Though in French it originally denoted a barrier implemented to stop the spread of disease, it has often been used in English in a metaphorical sense to refer to attempts to prevent the spread of an ideology deemed unwanted or dangerous, such as the containment policy adopted by George F. Kennan against the Soviet Union.

French Prime Minister Georges Clemenceau is credited with the first use of the phrase as a metaphor for ideological containment. In March 1919, he urged the newly independent border states (also called limitrophe states) that had seceded from Russian Empire and Soviet Russia to form a defensive union and thus quarantine the spread of communism to Western Europe; he called such an alliance a cordon sanitaire. This is still probably the most famous use of the phrase, though it is sometimes used more generally to describe a set of buffer states that form a barrier against a larger, ideologically hostile state. According to historian André Fontaine, Clemenceau's cordon sanitaire marked the real beginning of the Cold War: thus, it would have started in 1919 and not in 1947 as most historians contend it did.