Garter Principal King of Arms (also Garter King of Arms or simply Garter) is the senior king of arms and officer of arms of the College of Arms, the heraldic authority with jurisdiction over England, Wales and Northern Ireland. The position has existed since 1415.

Garter is responsible to the Earl Marshal for the running of the college. He is the principal adviser to the sovereign of the United Kingdom with respect to ceremonial and heraldry, with specific responsibility for England, Wales and Northern Ireland, and, with the exception of Canada, for Commonwealth realms of which Charles III is the sovereign. He also serves as the king of arms of the Order of the Garter and his seal and signature appear on all grants of arms made by the college.

On the death of the British monarch it is Garter's duty to proclaim the new monarch. Initially, the Accession Council meets at St James's Palace in central London to declare the new monarch from the deceased monarch's line. Once the new monarch has made a sacred oath before the council, Garter King of Arms steps out into the Proclamation Gallery which overlooks Friary Court to proclaim the new monarch.

The current Garter Principal King of Arms is David White.

William Bruges, the first Garter King of Arms, held the office by 1417. The exact date of his appointment is not known and no record of it survives. John Anstis discovered a royal warrant under the Privy Seal dated 22 May in the fifth year of the reign of Henry V (1417) wherein Bruges is called by his previous title, Guyenne King of Arms; the warrant orders that another be passed under the Great Seal of the Realm protecting Bruges while he travelled abroad with the king. Anstis then outlines two further pieces of evidence: (1) an instrument of 1422 or 1423 by which Bruges settled pension arrangements with the knights in which it is stated that he was appointed Garter at a previous full chapter meeting, and (2) a decree by Thomas of Lancaster, Duke of Clarence dated either 3 or 13 September 1417 which mentions "Garretier Roy d'armes des Anghis" As Henry V left for France on 27 July 1417, it can be deduced that Bruges must have been appointed in late July of that year.

This was accepted until Hugh Stanford London published evidence which appeared to date Bruges' appointment two years earlier than Anstis suggested. William Bruges' father, Richard, left a will dated 4 July 1415 and split into two parts: a testament dealing with his burial, charitable bequests and legacies to his wife, and a voluntas, which dealt with personal bequests. Although the testament (recorded in the London registry) makes no mention of anyone other than Richard's wife, the voluntas (copied in Archbishop Henry Chichele's registers) makes mention of his children. Reference is made to his son William, variously called "Gien", "Gyen" and "Gartere", and William's wife, called "Agnes Garter". The other register entries around the voluntas date to 1418 and 1419, so E. F. Jacob, the editor of a printed version of the register, suggests that the references to Garter may be a later gloss. But, Stanford London argued that later annotations would be consistent and refer to him as either Garter and Guyenne or simply Garter throughout, while Agnes would not have been called just Garter if it were a gloss. Instead, he suggests that Richard forgot to call his newly appointed son Garter at first and later included it alongside his old title in the will.

Criticising this point, Peter Begent finds no reference to Bruges being called anything but Guyenne or the equivalent title of Aquitaine King of Arms in records between 1415 and 1417, which is problematic for Stanford London's position. He argues that it is entirely possible that, if not a gloss, the voluntas was edited after 1415 as Richard Bruges included more bequests to his family.

The Garter Principal King of Arms was placed by King Henry V over all the whole body of heralds.