Osterwalde is a medieval shire (Gau) in the Eastphalia region of the Duchy of Saxony.
In present-day terms, it is located in northern Saxony-Anhalt.
Neighboring shires include Drevani, Chojna, Balsamgau, Nordthüringgau, Derlingau, and Bardengau.
By the 12th century, the Gau Osterwalde had become a part of the Margraviate of Brandenburg.
Together with the Balsamgau, it formed the westernmost part of Brandenburg, which later came to be known as the Altmark.