The Jomsvikings were a company of viking mercenaries of the 900s and 1000s, dedicated to the worship of such deities as Odin and Thor. Though staunchly pagan, they have been compared to the crusading orders of medieval Europe. However, they reputedly would fight for any lord able to pay their substantial fees. According to the Norse sagas (particularly the Jómsvíkinga saga, King Olaf Tryggvasson’s Saga, and stories found in the Flatey Book), their stronghold Jomsborg was located on the southern shore of the Baltic Sea, but the location is disputed by modern historians and archeologists. Nordisk familjebok asserts it to be located on the eastern side of the island of Wollin, on the hill Silberberg north of the town Wollin. Saxo Grammaticus mentioned settlement Julinum, inhabited by Slavic pirates, which is believed by many a reference to jomsvikings.

The existence of jomsvikings is however questioned by historians, because no contemporary chronicle mentions them; only later sagas.

The Saga of the Jomsvikings relates that the Jomsvikings were highly selective in deciding who to admit to their order. Membership was restricted to men of proven valor between 18 and 50 (with the exception of a boy named Vagn Åkesson, who defeated Sigvaldi Strut-Haraldsson in single combat at the age of 12.) In order to gain admission, prospective members were required to prove himself with a feat of strength, often taking the form of a ritual duel, or holmgang, with a Jomsviking.

Once admitted, the Jomsvikings required adherence to a strict code of conduct in order to instill a sense of military discipline among its members. Any violation of these rules could be punished with immediate expulsion from the order. Each Jomsviking was bound to defend his brothers, as well as to avenge their deaths if necessary. He was forbidden to speak ill of his fellows or to quarrel with them. Blood feuds between members were to be mediated by Jomsviking officers. Jomsvikings were forbidden to show fear or to flee in the face of an enemy of equal or inferior strength, though orderly retreat in the face of vastly outnumbering forces appears to have been acceptable. All spoils of battle were to be equally distributed among the entire brotherhood. No Jomsviking was permitted to be absent from Jomsborg for more than three days without the permission of the brotherhood. No women or children were allowed within the fortress walls, and none were to be taken captive. It is unclear, however, whether members were forbidden marriage or liaisons with women outside the walls.

Historians still debate the accuracy of the accounts of the Jomsvikings. Some maintain that the order was entirely legendary. The site of their headquarters has never been conclusively located, so confirming the tales of their exploits is somewhat difficult.

There are conflicting accounts of the origin of the order. One version states that the Jomsvikings were founded by Harald Bluetooth after his exile from Denmark. According to this version, he taught seafaring skills to the local Wends and led them on piratical raids against his enemies to the north. Jomsborg is described in these sources as a largely Wendish town with Norse officers. The Jómsvíkinga saga states that the settlement was entirely Norse, and that the brotherhood was founded by Palnatoke, who received the location from the Wendish ruler Burislav. Accounts of their size vary. Jomsborg is, in various sources, supposed to have held anywhere from 30 to 300 ships in its harbor.

Famous Jomsviking chieftains included Palnatoke, Sveyn Forkbeard, Sigvaldi Strut-Haraldsson, Thorkel the Tall, and Hemeng.

Styrbjarnar þáttr Svíakappa and Eyrbyggja saga relate that in the early 980s, Palnetoke lost the fortress and the control of the Jomsvikings to the exiled Swedish prince Styrbjörn the Strong. He allied with the Danish king Harald Bluetooth but brought the Jomsvikings to a devastating defeat against Styrbjörn's uncle Eric the Victorious at the Battle of the Fýrisvellir (Uppsala) in 984 or 985, when trying to take the crown of Sweden by force of arms. The fact that the Jomsvikings lost was attributed to a pact that king Eric made with Odin. Two runestones (DR 295 and DR 279) from this time relate of men who died with honour at Uppsala, probably two Jomsvikings.

The Battle of Svolder, at which the Jomsvikings betrayed Olaf Trygvasson. (painting by Otto Sinding).Jómsvíkinga saga tells that in 986, they attacked Haakon Jarl in Norway and were defeated in the Battle of Hjörungavágr. After these two decisive defeats, the power of the Jomsvikings waned, but Olaf Trygvasson's Saga relates that they played a decisive, if treacherous, role in the Battle of Svolder in 1000. At Svolder, a Jomsviking force led by Sigvald Jarl abandoned King Olaf of Norway and joined forces with his enemies to annihilate his fleet. This action may have been intended to forstall the Christianization of Scandinavia, of which Olaf was a stalwart supporter.

Jomsvikings are also reported to have raided eastern England in 1009, and made forays into various Scandinavian territories during the early 1000's. Around 1013 the Jomsvikings were campaigning in England on behalf of Sveyn Forkbeard, but were bribed by the English and switched sides to fight for Ethelred the Unready. Their decline continued over the next few decades. In 1043, according to the Heimskringla, Magnus I of Norway decided to put an end to the Jomsviking threat. He sacked Jomsborg, destroyed the fortress and put the surviving bretheren to death.

Archaeological evidence
Runestones are counted as historic documents about the events of the Viking Age in Scandinavia. The following three runestones probably mention Jomsvikings who died with Styrbjörn the Strong south of Uppsala. Note that the first runestone mentions a warleader named Toki Gormsson and he may be a son of the Danish king Gorm, an interpretation which fits the fact that Styrbjörn was allied with another son of Gorm, Harald Bluetooth.

The runestone DR 295 in Hällestad, Hallandia says: Eskil raised this stone after Toki Gormsson, his beloved warleader. He did not flee at Uppsala. Champions erected this stone after their brother on the hill. They went closest with Toki.
A : askil : sati : stin : þansi : ift[iR] : tuka : kurms : sun : saR : hulan : trutin : saR : flu : aigi : at : ub::salum
B satu : trikaR : iftiR : sin : bruþr stin : o : biarki : stuþan : runum : þiR :
C (k)(u)(r)(m)(s) ( (t)(u)(k)(a) : kiku : (n)(i)(s)(t)[iR]
A Æskel satti sten þænsi æftiR Toka Gorms sun, seR hullan drottin. SaR flo ægi at Upsalum
B sattu drængiaR æftiR sin broþur sten a biargi støþan runum. ÞeR
C Gorms Toka gingu næstiR.
The runestone DR 279 in Sjörup, Scania, relates: He did not flee at Uppsala, but fought as long as he had weapons.
[+ sa]ksi : sati : st[in] : þasi : huftiR : o[s]biurn : (s)in : fil(a)go ' (t)u-a[s : sun :] saR : flu : aki : a[t :] ub:sal(u)m : an : ua : maþ : an : uabn : a(f)þi '
Saxi satti sten þæssi æftiR Æsbiorn, sin felaga, To[f]a/To[k]a sun. SaR flo ægi at Upsalum, æn wa mæþ han wapn hafþi.
On the Högby Runestone, it says The good freeman Gulli had five sons. The brave champion Asmund fell on the Fyris.
In fiction
Guy Gavriel Kay's novel Last Light of the Sun deals with a number of fictional nations each modeled after a historical culture in our own worlds. One such nation, the Erlings, is based on early medieval Scandinavians. The Erlings have a renowned band of mercenaries based at "Jormsvik" whose code and culture are clearly based on the Jomsvikings.