A plenipotentiary (from the Latin plenus "full" and potens "powerful") is a diplomat who has full powers—authorization to sign a treaty or convention on behalf of a sovereign. When used as a noun more generally, the word can also refer to any person who has full powers. As an adjective, it describes something which confers full powers, such as an edict or an assignment.

Before the era of rapid international transport or essentially instantaneous communication (such as telegraphy in the mid-19th century and then radio), diplomatic mission chiefs were granted full (plenipotentiary) powers to represent their government in negotiations with their host nation. Conventionally, any representations made or agreements reached with a plenipotentiary would be recognized and complied with by their government.

Historically, the common generic term for high diplomats of the crown or state was minister. It therefore became customary to style the chiefs of full ranking missions as minister plenipotentiary. This position was roughly equivalent to the modern ambassador, a term that historically was reserved mainly for missions between the great powers and also relating to the city-state of Venice.

Permanent missions at a bilateral level were chiefly limited to relations between large, neighboring or closely allied powers, rarely to the very numerous small principalities, hardly worth the expense. Diplomatic missions were dispatched for specific tasks, such as negotiating a treaty bilaterally, or via a conference of plenipotentiaries, such as the Imperial Diet of the Holy Roman Empire. In such cases, it was normal to send a representative minister empowered to cast votes. For example, in the Treaty of Paris, ending the American Revolution, John Adams, Benjamin Franklin and John Jay were named "minister plenipotentiary of the United States" to the Netherlands, France and Spain, respectively.

By the time of the Vienna Congress (1814–15), which codified diplomatic relations, ambassador had become a common title, and was established as the only class above minister plenipotentiary. It gradually became the standard title for bilateral mission chiefs, as their ranks no longer tended to reflect the importance of the states, which came to be treated as formally equal.

In modern times, heads of state and of government, and more junior ministers and officials, can easily meet or speak with each other personally. Therefore, ambassadors do not require plenipotentiary powers, even though they are designated and accredited as "extraordinary and plenipotentiary".

Besides diplomatic plenipotentiaries, some permanent administrators are also given plenipotentiary powers. Central governments have sometimes conferred plenipotentiary status (either formally or de facto) on territorial governors. This has been most likely to occur when the remoteness of the administered territory made it impracticable for the central government to maintain and exercise its policies, laws and initiatives directly. In modern times, the plenipotentiary title has sometimes been revived; for example, for the administrators of protectorates, or in other cases of indirect rule.

Examples of plenipotentiary administration are given below.