John I of Brandenburg

John I (c. 1213–1266) was the elder son of Margrave Albert II and ruled Brandenburg jointly with his brother Otto III from 1220, in person after 1225. Their reign enlarged the margraviate with the Uckermark, Stargard, the Lubusz Land and the first Neumark lands beyond the Oder, and they promoted Cölln and Berlin.

In 1230 he married Sophie, daughter of King Valdemar II of Denmark, and between 1261 and 1264 held King Eric V of Denmark prisoner. In 1266 the brothers founded Chorin Abbey.