The body of ambassadors, envoys and negotiators who represent the United Federation of Planets to other powers, from the Khitomer peace conference of 2293 to the reunification talks with Ni'Var in the 32nd century.
The Federation Diplomatic Corps conducts the Federation's negotiations with foreign governments. Its members take part in the Khitomer Conference in Star Trek VI: The Undiscovered Country, and a regulation glimpsed in The Next Generation episode The Measure of a Man gives it special authority over the transfer of Starfleet officers.
In the Deep Space Nine episode Broken Link a contact in the corps, an old friend of Curzon Dax, warns Jadzia that worlds along the Klingon border are pressing for a pre-emptive strike. In Star Trek: Insurrection the corps is so absorbed by the Dominion War that the Enterprise-E is pressed into diplomatic errands, to Picard's irritation.
The corps endures into the 32nd century, when Admiral Vance informs it that Michael Burnham will speak for the Federation on Ni'Var in Discovery's Unification III.