Otto III, nicknamed the pious (1215 – 9 October 1267 in Brandenburg an der Havel) is Margrave of Brandenburg jointly with his elder brother John I until John dies in 1266.
Otto III then rules alone, until his death, the following year.
The reign of these two Ascanian Margraves is characterized by an expansion of the Margraviate, which annexes the remaining parts of Teltow and Barnim, the Uckermark, the Lordship of Stargard, the Lubusz Land and parts of the Neumark east of the Oder.
They consolidate the position of Brandenburg within the Holy Roman Empire, which is reflected in the fact that in 1256, Otto III is a candidate to be elected King of the Germans.
They found several cities and develop he twin cities of Cölln and Berlin.
They expand the Ascanian castle in nearby Spandau and make it their preferred residence.
Before their death, they divide the Margraviate in a Johannine and an Ottonian part.
The Ascanians are traditionally buried in the Lehnin Abbey in the Ottonian part of the country.
In 1258, they found a Cistercian monastery named Mariensee, where members of the Johannine line could be buried.
In 1266, they change their mind and found a second monastery, named Chorin, 8 km southwest of Mariensee.
John is initially buried at Mariensee; his body is moved to Chorin in 1273.
After the Ottonian line dies out in 1317, John I's grandson Waldemar reunites the Margraviate.
