Lothair III, sometimes numbered Lothair II and also known as Lothair of Supplinburg (c. June 1075 – 4 December 1137), was Holy Roman Emperor from 1133 until his death. He was appointed Duke of Saxony in 1106 and elected King of Germany in 1125 before being crowned emperor in Rome. The son of the Saxon count Gebhard of Supplinburg, his reign was troubled by the constant intriguing of the Hohenstaufens, Duke Frederick II of Swabia and Duke Conrad of Franconia. He died while returning from a successful campaign against the Norman Kingdom of Sicily.

In 1013, a certain Saxon nobleman named Liutger was mentioned as a count in or of the Harzgau subdivision of Eastphalia. His grandson Count Gebhard, father of Emperor Lothair, possibly acquired the castle of Süpplingenburg about 1060 via his marriage with Hedwig, a daughter of the Bavarian count Frederick of Formbach and his wife Gertrud, herself a descendant of the Saxon margrave Dietrich of Haldensleben who secondly married the Billung duke Ordulf of Saxony upon Count Frederick's death.

Little is known of Lothair's youth. His name first appears in the contemporary records in 1088. His father Gebhard of Supplinburg joined the Saxon rebellion against the ruling Salian dynasty and died on 9 June 1075 in the Battle of Langensalza, fighting troops loyal to emperor Henry IV. Shortly after Gebhard's death Lothair was born at Unterlüß. In 1107 he married Richenza, daughter of Count Henry of Northeim and Gertrude of Brunswick, heiress of the Brunonids.

Lothair's land purchases, inheritance and marriage alliances among the Saxon nobles, resulted in the acquisition of the domains of the House of Billung and the Counts of Northeim. The marriage with Richenza of the Brunonids in particular, made him the wealthiest nobleman among his fellow Saxons. He supported future emperor Henry V during his 1104 rebellion against his father Henry IV, and the ensuing disempowerment campaign, that culminated in the abdication of the emperor on December 31, 1105, and his son's coronation a few days later. For his loyalty Lothair was rewarded with the fief of title and estate of the Duchy of Saxony upon the death of duke Magnus of Billung, who had died without an heir in 1106. Emboldened by the promotion and incensed over the king's increasingly autocratic rule, such as the wanton imposition of a new tax on ducal lords, Duke Lothair joined the growing opposition party to Henry. He acted autonomously by vesting Count Adolf of Schauenburg with the newly established County of Holstein in 1111. Lothair was temporarily deposed in 1112, when Henry transferred the ducal title to Otto of Ballenstedt. He was soon reinstated when count Otto fell into disgrace and he tactically submitted himself to Henry V. In 1115 however, he took command of the rebellious Saxon forces and defeated the emperor in the Battle of Welfesholz. Henry completely lost control over the administration and the revenue of Saxony. When in 1123 Henry V vested Count Wiprecht of Groitzsch with the Margraviate of Meissen, Lothair enforced the appointment of Conrad of Wettin and ceded the March of Lusatia to Count Albert the Bear.

Upon Emperor Henry V's death in 1125, Archchancellor Adalbert summoned the royal electoral assembly in Mainz. On August 24 the electors declined the candidacy of the primary contender Duke Frederick of Hohenstaufen, who destroyed his chances due to his appalling overconfidence (ambicone cecatus) and his refusal to accept free princely elections (libera electio). Adalbert of Mainz considered Lothair to be a suitable candidate. Although the most powerful territorial prince in Saxony, he was of advanced age (slightly over fifty years of age) and had no male heir, not the ideal prerequisites for a long dynastic line of kings. He was elected King of Germany and asserted himself against Leopold III of Austria and Charles the Good. His election was notable in that it marked a departure from the concept of hereditary succession as the electors preferred a sovereign with moderate powers after the Salian era of oppressio. Somewhat naive concerning the complex power struggle between the papacy and the empire, Lothair also consented to several symbolic acts that were subsequently interpreted by the Roman Curia as signaling acceptance of papal confirmation of his position.

Duke Vladislaus I of Bohemia died in 1125. The succession was disputed among his surviving brother Soběslav I and his Moravian cousin Otto the Black, who was supported by Vladislaus' widow Richeza of Berg. In late 1125 Lothair joined Otto's side, who had advanced large sums of money.

A military campaign against Soběslav was launched and in February 1126 Lothair's force entered Bohemian territory and was promptly defeated at the Battle of Chlumec. Soběslav captured high-ranking nobles, like Albert the Bear and Louis I of Thuringia. However, Soběslav immediately went to meet Lothair at his camp and formally requested and received the fief of Bohemia. Peace was restored, prisoners set free and although the winner of the battle had submitted himself to the losing side, he secured full legitimacy and lasting prestige.

Having both Saxon and Bavarian ancestry, the Supplinburg dynasty was a political opponent of the Salian dynasty and the House of Hohenstaufen. Disputes arose with Duke Frederick II when he refused to hand over property to Lothair, which the king considered to be royal property, the Staufer on the other hand argued, that it belonged to the Salian heritage. The contentious assets had long been administered together with other Salian domestic estates, their origin was hard to determine and difficult to separate. Lothair advocated the principle that all of the assets in question had now become imperial properties due to the extinction of the Salian dynasty. The first armed engagements between Lothair and the Staufer took place as early as 1125 and increased in the years that followed. Lothair, with the approval obtained at a meeting of the princes in Regensburg, attempted to seize the crown lands, which provoked a Staufer reaction. Lothair then isolated Frederick II as he placed him under Imperial ban and withdrew the Franconian ducal fief from Conrad.