The Gallowglass (also spelled galloglass, gallowglas or galloglas; from Irish: gallóglaigh meaning "foreign warriors") were a class of elite mercenary warriors who were principally members of the Norse-Gaelic clans of Ireland and Scotland between the mid 13th century and late 16th century. It originally applied to Scots, who shared a common background and language with the Irish, but as they were descendants of 10th-century Norse settlers who had intermarried with the local population in western Scotland, the Irish called them Gall Gaeil ("foreign Gaels").

An early family of gallowglasses was the MacSweeneys, settled by the O'Donnells in north Donegal. These were followed by MacDonnells, MacAlisters, MacCabes and several other groups settled by powerful Irish nobles in different areas. The gallowglasses were attractive as heavily armoured, trained infantry to be relied upon as a strong defence for holding a position, unlike most Irish foot soldiers, who were less well armoured than the typical Irish noble who fought as cavalry.

They were a significant part of Irish infantry before the advent of gunpowder, and depended upon seasonal employment by Irish clan chiefs. An Irish military leader would often choose a gallowglass to serve as his personal aide and bodyguard because, as a foreigner, a gallowglass would be less influenced by the internal feuds over dynastic succession within the derbhfine of the Irish clans.

The Irish language gallóglach is derived from gall "foreign" and óglach; from Old Irish oac (meaning "youth") and Old Irish lóeg (meaning "calf" but later becoming a word for a "hero"). The Old Irish language plural gallóglaigh is literally "foreign young warriors".(The modern Irish plural is galloglagh.)

According to the Oxford English Dictionary (OED), the English language singular, gallowglass, is an anglicisation of Old Irish plural gallóglaigh, rather than the singular. The OED, which prefers the spelling "galloglass", provides examples of various English plural forms, dating back to a c. 1515 use of "galloglasseis". The OED states that the conventional English plural, gallo(w)glasses, is "probably the result" of the duplicate, redundant plural suffix -es being added to the Irish plural. (The OED notes that while the modern Irish plural galloglagh appears to be "etymologically correct", it is recorded later than English gallowglasses.) For instance, Shakespeare uses the form "gallowglasses" in Macbeth. The OED adds that "galloglasses is found ... in our earliest quot," even though "in some early instances", a relatively early, alternate English plural form, galloglas, is also used. Hence, English gallowglass is probably a singularised form, which post-dates the plural, gallowglasses.

The gallowglass were from the western coast of Scotland, principally Argyll and the Western Isles. Their weapons were swords and axes. Each was usually accompanied by a man to take care of his weapons and armour and a boy to carry provisions.

A description from 1600 speaks of the gallowglass as "pycked and seelected men of great and mightie bodies, crewell without compassion. The greatest force of the battell consisteth in them, chosinge rather to dye then to yeelde, so that when yt cometh to handy blowes they are quickly slayne or win the feilde".

The first record of gallowglass service was in 1259, when Aedh Ó Conchobair, King of Connacht, received a dowry of 160 Scottish warriors from the daughter of Dubhghall mac Ruaidhrí, the King of the Hebrides. They were organised into groups known as a corrughadh, which consisted of about 100 men.