An ambassador is an official appointed by the government of a nation-state to represent its interests, usually to another country or a foreign government, but sometimes to an international organisation such as the United Nations. An ambassador is the highest-ranking official in an embassy or other diplomatic post. Because the ambassador represents the government, and sometimes the country itself, diplomatic protocol dictates that an ambassador be addressed respectfully, using terms similar to those used to address a head of state.
Greet an ambassador with the formal title, "Your Excellency."
Use the title "Ambassador" after being introduced, or in more casual conversation.
Introduce an ambassador with the full formal title: "His (or Her) Excellency, Mr. (or any other appropriate title) Firstname Lastname, the [Country] Ambassador. If the ambassador holds a title such as "Doctor," "Father," or "General," use that instead if "Mr." or "Mrs." Use the adjectival form of the country's name (i.e. French, German). Do not use "Excellency" if the ambasssador has the royal title "His/Her Royal Highness" or "His/Her Serene Highness." In this case, use only the royal title, with the ambassador designation after the given name.
Address a U.S. ambassador as "Mr. Ambassador" or "Madam Ambassador" directly, and refer to them as "The Honorable ..." in the third person. U.S. ambassadors do not use the "Excellency" title.
When writing to a U.S. ambassador stationed in the Western Hemisphere, use the reference "Ambassador of the United States of America." When writing to a U.S. ambassador stationed outside the Western Hemisphere, use the reference "American Ambassador."