Lettres de cachet (French: [lɛtʁ də kaʃɛ]; lit. '"letters of the sign/signet"') were letters signed by the king of France, countersigned by one of his ministers, and closed with the royal seal. They contained orders directly from the king, often to enforce arbitrary actions and judgments that could not be appealed.

In the case of organized bodies, 'lettres de cachet’ were issued for the purpose of preventing assembly or accomplishing some other definite act. The provincial estates were convoked (called to assembly) in this manner, and it was by a lettre de cachet (in this case, a lettre de jussipri), or by showing in person in a lit de justice, that the king ordered a parlement to register a law despite that parlement's refusal to pass it.

The best-known lettres de cachet, however, were penal, by which a subject was imprisoned without trial and without an opportunity of defense (after inquiry and due diligence by the lieutenant de police) in a state prison or an ordinary jail, confinement in a convent or the General Hospital of Paris, transportation to the colonies, or expulsion to another part of the realm, or from the realm altogether. The lettres were mainly used against drunkards, troublemakers, prostitutes, squanderers of the family fortune, or insane persons. The wealthy sometimes petitioned such lettres to dispose of inconvenient individuals, especially to prevent unequal marriages (nobles with commoners), or to prevent a scandal (the lettre could prevent court cases that might otherwise dishonour a family).

In this respect, the lettres de cachet were a prominent symbol of the abuses of the ancien régime monarchy, and as such were suppressed during the French Revolution. In 1789 and 1790, all cases were reviewed by a commission which confirmed most of the sentences. Historian Claude Quétel has interpreted these confirmations as indicating that the lettres were not as arbitrary and unjust as they have been represented after the Revolution, and he hence speaks of a Légende noire.