The Knight of Glin, also known as the Black Knight or Knight of the Valley, was an hereditary title held by the FitzGerald and FitzMaurice families of County Limerick, Ireland, since the early 14th century. The family was a branch of the FitzMaurice/FitzGerald Dynasty commonly known as the Geraldines and related to the now extinct Earls of Desmond who were granted extensive lands in County Limerick by the Crown. The title was named after the village of Glin, near the Knight's lands. The Knight of Glin was properly addressed as "Knight".

The FitzGerald dynasty is a Hiberno-Norman noble and aristocratic dynasty, originally of Cambro-Norman and Anglo-Norman origin. They have been peers of Ireland since at least the 13th century, and are described in the Annals of the Four Masters as having become "more Irish than the Irish themselves" or Gaels, due to assimilation with the native Gaelic aristocratic and popular culture. The dynasty has also been referred to as the Geraldines and Ireland's largest landowners. They achieved power through colonisation and the conquest of large swathes of Irish territory by the sons and grandsons of Gerald de Windsor. Gerald de Windsor was the first Castellan of Pembroke Castle in Wales, and became the male progenitor of the FitzMaurice and FitzGerald Dynasty. His father, Baron Walter FitzOther, was the first Constable and Governor of Windsor Castle for William the Conqueror, and was the Lord of 38 manors in England, making the FitzGeralds one of the "service families" on whom the King relied for his survival. Some of its members became the Black Knights, Green Knights and White Knights.