The Hassegau was an early medieval shire (Gau) in the Eastphalia region of the Duchy of Saxony. It was located in the corner between the Saale (to the east), Unstrut (to the south), and Wipper (to the north) rivers; its most important town was Merseburg. In present-day borders, it is in the southeastern part of Saxony-Anhalt.
The Hassegau is bordered by the following shires (from the north, clockwise): Schwabengau, Nudici (Slavic), Chutizi (Slavic), Weitaha (Slavic), Engilin, Friesenfeld. The Friesenfeld is considered a distinct shire by some sources, but in other sources it is considered part of the Hassegau. The meaning of the name Hassegau is unclear; but it may be derived from Hesse, since several nearby shires have names that are clearly derived from other distant Germanic tribes (Schwabengau, Friesenfeld, Engilin). Possibly, these names signify the tribes that colonized the areas.
In the 10th century, the County Palatine of Saxony was founded in parts of the Hassegau. In 968, the Bishopric of Merseburg was founded, which had as temporal property inside the Hassegau only the town of Merseburg itself. In the northern part of the shire, the County of Mansfeld established itself in the 11th century. By the year 1200, the shire had completely disintegrated, and apart from the mentioned states, parts of it belonged to the Archbishopric of Magdeburg, the Lordship of Querfurt, and the Margraviate of Landsberg.
Counts of the Hassegau included members of the houses of Wettin and Mansfeld, among others.