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 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 to accomplish 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, confined in a convent or the General Hospital of Paris, transported to the colonies, or expelled 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.

The power to issue lettres de cachet was a royal privilege recognized by the French monarchic civil law that developed during the 13th century, as the Capetian monarchy overcame its initial distrust of Roman law. The principle can be traced to a maxim which furnished a text of the Pandects of Justinian: in their Latin version, "Rex solutus est a legibus", or "The king is released from the laws". "The French legal scholars interpreted the imperial office of the Justinian code generically and arrived at the conclusion that every 'king is an emperor in his own kingdom', that is, he possesses the prerogatives of legal absolutism that the Corpus Juris Civilis attributes to the Roman emperor."

This meant that when the king intervened directly, he could decide without heeding the laws, and even contrary to the laws. This was an early conception, and in early times the order in question was simply verbal; some letters patent of Henry III in 1576 state that François de Montmorency was "prisoner in our castle of the Bastille in Paris by verbal command" of the late king Charles IX.

In the 14th century, the principle was introduced that the order should be written, and hence arose the lettre de cachet. The lettre de cachet belonged to the class of lettres closes, as opposed to lettres patentes, which contained the expression of the legal and permanent will of the king, and had to be furnished with the seal of state affixed by the chancellor.

The lettres de cachet, on the contrary, were signed simply by a secretary of state for the king; they bore merely the imprint of the king's privy seal, from which circumstance they were often called, in the 14th and 15th centuries, lettres de petit signet or lettres de petit cachet, and were entirely exempt from the control of the chancellor.