Costello is a surname in the English language.
The surname has been borne by a notable Irish family who claimed descent from Jocelyn de Angulo (fl. 1172), an Anglo-Norman knight. The family first appears on record in Ireland in 1193, when the Annals of the Four Masters state: Inis Clothrann do orgain la macaibh Oisdealb, & la macaibh Conchobhair Maonmaighe. Inishcloghbran was plundered by the sons of Osdealv, and the sons of Conor Moinmoy.
Oistealb or Osdealv was the Gaelic rendering of Jocelyn; the sons of Joycelyn were Philip, Gilbert, and William de Angulo. The descendants of William de Angulo/Mac Costello settled in Connacht and became Gaelicized, dropping the surname de Angulo in favor of Mac Oisdealb, or Mac Costello. The barony of Costello in east Mayo is named from this family.
The name Costello has a superficially Italian appearance to it, though it is not actually of Italian origin. However, it has occasionally been adopted as a pseudonym or stage name by famous people of Italian descent, including Al Costello (né Giacomo Costa), Frank Costello (né Francesco Castiglia) and Lou Costello (né Louis Francis Cristillo).