Jarl (/jɑːl/) was a rank of the nobility in Scandinavia during the Viking Age and Early Middle Ages. The institution evolved over time and varied by region. In Old Norse, it meant "chieftain", specifically one appointed to rule a territory in a king's stead. It could also denote a sovereign prince.^{[citation needed]} For example, during the Viking age, the rulers of several of the petty kingdoms of Norway held the title of jarl, often wielding no less power than their neighboring kings. In later medieval Sweden and Norway, there was typically only one jarl in the kingdom, second in authority only to the king. The title became obsolete in the Middle Ages and was replaced by the rank of duke (hertig/hertug/hertog). The word is etymologically related to the English earl.

The term jarl (Old Norse: jarl, Old Swedish: iarl, iærl, Old Danish: jærl) has been connected to various similar words across Germanic languages, such as Proto-Norse eril, Old English eorl (meaning warrior, hero, or chieftain, related to modern English earl), Old Saxon erl (man, nobleman), and Old High German erl- in personal names such as Erlaberaht.

Some scholars have suggested that jarl shares a connection with the Old Germanic erilaz and the Latin heruli or erular, terms that once referred to a Germanic tribe who, according to some scholars, once lived on Danish islands and the Skåne region (see Heruli). However, this connection has been dismissed by others.

The word jarl is known in Norway from the scaldic poem Háleygjatal, dating from the late 9th century.

In Eddic poetry, a jarl is often a free man of good standing, but not necessarily of high office. The concept of the jarl also appears in the poem Rígsþula. According to the myth, the god Ríg fathers three sons—Thrall, Karl, and Jarl—who become the ancestors of the social classes: thralls, peasants, and the warrior nobility. Jarl marries Erna, daughter of hersir (local chieftain), and their youngest son is named Konr or Konr ungr (king). Based on Rígsþula, German historian Konrad Maurer traced the concept of jarl back to the legendary Norwegian king Harald Fairhair (c. 850 – c. 932). During those times, jarl would have meant a member of the aristocratic class. This is supported by Old English laws, in which eorl refers to a noble person. However, some scholars date the composition of Rígsþula to 12th century or later, in which case it cannot be considered a good source for a 10th century concept.

During the Merovingian period, jarl developed into an office of independent chieftains. According to the Saga of Harald Fairhair, King Harald set a jarl in each county to collect taxes:

King Haraldr made this law everywhere he established his dominion over, that he took possession of all inherited property and made all farmers pay him land dues, both rich and poor. He set a jarl in every district who was to administer the laws of the land and collect fines and land dues, and the jarl was to have one third of the taxes and dues for his maintenance and expenses. Each jarl was to have under him four or more lords [hersir] and each of them was to have revenues of twenty marks. Each jarl was to provide the king with sixty fighting men for his army, and each lord twenty men. And so much had King Haraldr increased the taxation and land dues that his jarls had more power than kings had had in the past. And then, when this was learned in Þrándheimr, many of the ruling class went to see King Haraldr and became his men.

— Haralds saga ins hárfagra, Chapter 6 [translation by Alison Finlay and Anthony Faulkes]