An ambassador is an official envoy, especially a high-ranking diplomat who represents a state and is usually accredited to another sovereign state or to an international organization as the resident representative of their own government or sovereign or appointed for a special and often temporary diplomatic assignment. The word is also used informally for people who are known, without national appointment, to represent certain professions, activities, and fields of endeavor, such as sales.

An ambassador is the ranking government representative stationed in a foreign capital or country. The host country typically allows the ambassador control of specific territory called an embassy, whose territory, staff, and vehicles are generally afforded diplomatic immunity in the host country. Under the Vienna Convention on Diplomatic Relations, an ambassador has the highest diplomatic rank. Countries may choose to maintain diplomatic relations at a lower level by appointing a chargé d'affaires in place of an ambassador.

The equivalent to an ambassador exchanged among members of the Commonwealth of Nations are known as High Commissioners. The ambassadors of the Holy See are known as Papal or Apostolic Nuncios.