Killian or Kilian, as a given name, is an Anglicized version of the Irish name Cillian. The name Cillian was borne by several early Irish saints including missionaries to Artois and Franconia and the author of the life of St Brigid.

The name is said to derive from Saint Kilian, an Irish missionary to Germany in the 7th century, who, according to the Acta Sanctorum, was born in Mullagh, County Cavan, Ireland c. 640. He departed for his mission to the continent with 12 apostles from Kilmacologue in the parish of Tuosist, County Kerry, Ireland. In 689 he was martyred in Würzburg, Franconia, Germany, and subsequently became the city's patron saint.

The most likely meaning of the name is "little church", a reference to someone prayerful or spiritual, cill meaning "church" in Gaelic while the suffix "-ín" is used affectionately to indicate a 'pet' or diminutive status. The Rev. Patrick Woulfe wrote that Cillian is the 'pet' diminutive of Ceallach which means "war", "strife" or "bright-headed".