The Saxon Eastern March or Ostmark (German: Sächsische Ostmark) was a march of the Holy Roman Empire from the 10th until the 12th century. The term "eastern march" or "ostmark" comes from the Latin term marchia Orientalis and originally could refer to either a march created on the eastern frontier of the Duchy of Saxony or another on the eastern border of the Duchy of Bavaria: the Bavarian Eastern March, corresponding to later Austria.
The Ostmark was created out of the vast Marca Geronis of Gero after his death in 965. As Gero had died without issue his lands were divided by the establishment of the Northern March between the Elbe and Oder rivers, while the remaining Saxon Eastern March consisted of the southern territory between Saale and Bóbr, roughly corresponding to the modern German Free State of Saxony. Emperor Otto I invested Odo I with the title of a margrave.
At the same time the March of Meissen, the March of Merseburg and the March of Zeitz were created in the southern half of the Ostmark. In 1002 Odo's successor Gero II lost the eastern part of the march to Bolesław I of Poland, nevertheless Bolesław's son Mieszko II had to return the conquered territory to Emperor Conrad II in 1031. In 1046 Dedi I from the House of Wettin inherited the march, his son and successor Henry I was in addition granted the March of Meissen by Emperor Henry IV in 1089. Both marches remained under Wettin administration and later became the nucleus of the Saxon Electorate.
After the Margraviate of Landsberg and the March of Lusatia were split off from it, the remaining areas were united with the Margraviate of Meissen in 1123. The last time that the Ostmark and Lusatia appear separate is when the former was received by Henry of Groitzsch in 1128 and the latter was reserved until 1131. Henry however did not prevail and by 1136 the march had fallen back to Conrad of Meissen. During the various divisions of the Wettin lands, the territory was split up several times; most of it belonged to the Ernestine duchies.
The term Osterland is still used today to describe the historic region which was at the centre of the march. While the borders of the Ostmark changed frequently, in modern times the term is generally understood to mean the area between the rivers Saale and Mulde.