Letters patent are a type of legal instrument in the form of an open letter issued by a monarch or government, granting an office, right, monopoly, title, or status to a person or to some entity such as a corporation. The opposite of letters patent (Lat. litterae patentes) is letters close (Lat. litterae clausae), which are personal in nature and sealed so that only the recipient can read their contents. Note that due to the Latin idiom involved, a single document is not a "letter patent" but still "letters patent."

Letters patent often start with a salutation such as "To all to whom these presents shall come Greeting" or "To all to whom these presents shall come or whom the same may in any way concern, GREETING:" or even just "To all and singular, greeting." However, a document starting with such a clause may merely be a deed poll. Letters patent can be used for the granting of city status or coats of arms, for the creation of corporations, or by a monarch to create an office. They are also common in printed diplomas and academic degrees from educational institutions.

A particular form of letters patent has evolved into the modern patent granting exclusive rights in an invention.

In the United Kingdom and some Commonwealth realms (where letters patent may be issued by the Governor-General), letters patent are issued under the Royal Prerogative and constitute a rare, if significant, form of legislation without the consent of Parliament. Letters patent may also be used to grant royal assent to acts of Parliament.

In the United States, the forgery of letters patent granted by the President of the United States is a crime subject to fine and/or imprisonment up to ten years ( ). Without letters patent a person is unable to assume an appointed office. Such prompted the Marbury v. Madison suit where William Marbury and three others petitioned the US Supreme Court to order James Madison to deliver their letters for appointments made under the previous administration.