| Otto III | |
|---|---|
 Monument to John I (sitting) and his brother Otto III in the Siegesallee in Berlin, by Max Baumbach. | |
| Margrave of Brandenburg | |
| Reign | 1220–1267 |
| Predecessor | Albert II |
| Successor | Otto IV |
| Born | 1215 |
| Died | 9 October 1267 Brandenburg an der Havel |
| Burial | Church of the Dominican monastery in Strausberg |
| Spouse | Beatrice of Bohemia |
| Issue | John III, Margrave of Brandenburg-Salzwedel Otto V, Margrave of Brandenburg-Salzwedel Albert III, Margrave of Brandenburg-Salzwedel Otto VI, Margrave of Brandenburg-Salzwedel Kunigunde, Duchess of Slavonia and Limburg Matilda, Duchess of Pomerania |
| House | House of Ascania |
| Father | Albert II, Margrave of Brandenburg |
| Mother | Matilda of Lusatia |
Otto III, nicknamed the pious (1215 – 9 October 1267 in Brandenburg an der Havel) was Margrave of Brandenburg jointly with his elder brother John I until John died in 1266. Otto III then ruled alone, until his death, the following year.
The reign of these two Ascanian margraves was characterized by an expansion of the margraviate, which annexed 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 consolidated the position of Brandenburg within the Holy Roman Empire, which was reflected in the fact that in 1256, Otto III was a candidate to be elected King of the Germans. They founded several cities and developed the twin cities of Cölln and Berlin. They expanded the Ascanian castle in nearby Spandau and made it their preferred residence.
Before their death, they divided the margraviate in a Johannine and an Ottonian part. The Ascanians were traditionally buried in the Lehnin Abbey in the Ottonian part of the country. In 1258, they founded a Cistercian monastery named Mariensee, where members of the Johannine line could be buried. In 1266, they changed their mind and founded a second monastery, named Chorin, 8 km southwest of Mariensee. John was initially buried at Mariensee; his body was moved to Chorin in 1273.
After the Ottonian line died out in 1317, John I's grandson Waldemar reunited the margraviate.