The Royal Guard Cap is an item from The Legend of Zelda: Breath of the Wild. It is a thematic head armor originally worn by the Royal Guard one hundred years before Breath of the Wild. It is one of the three pieces of the "The Royal Guard Uniform" set along with the Royal Guard Uniform and Boots.
It can be obtained during the Side Quest "EX Royal Guard Rumors" upon "The Champions' Ballad" DLC pack 2 installation. It can be found in an EX Treasure Chest in front of a knight statue on the second floor of the Sanctum in Hyrule Castle above the boss room where Calamity Ganon resides. This floor can be identified by a unique pool of Malice in the center with vein like tendrils growing out of it. Entering this floor is safe as no boss fights are triggered and the malice pool in the room can be easily avoided. However Link must climb the outside of the Sanctum to reach it. The second floor is considered to be part of the Sanctum.
Though promotional artwork depicts the Crest of the Royal Family of Hyrule on top of the cap this feature is absent in-game.