| John I | |
|---|---|
 Monument to John I (sitting) and his brother Otto III in the Siegesallee in Berlin, by Max Baumbach. | |
| Margrave of Brandenburg | |
| Reign | 1220–1266 |
| Predecessor | Albert II |
| Successor | Otto III |
| Born | c. 1213 |
| Died | 4 April 1266 |
| Burial | Mariensee monastery |
| Spouse | Sophie of Denmark Brigitte of Saxony |
| Issue | John II, Margrave of Brandenburg-Stendal Otto IV, Margrave of Brandenburg-Stendal Conrad, Margrave of Brandenburg-Stendal Eric, Archbishop of Magdeburg Helene, Margravine of Landsberg Hermann, Bishop of Havelberg Agnes, Queen of Denmark Henry I, Margrave of Brandenburg-Stendal Matilda, Duchess of Pomerania Albert of Brandenburg |
| House | House of Ascania |
| Father | Albert II, Margrave of Brandenburg |
| Mother | Matilda of Lusatia |
John I, Margrave of Brandenburg ( c. 1213 – 4 April 1266) was from 1220 until his death Margrave of Brandenburg, jointly with his brother Otto III "the Pious".
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 into a Johannine and Ottonian parts. 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 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.