Stephen I. (March 14, 1271 – December 10, 1310) was the duke of Lower Bavaria from 1290 until 1310 as co-regnant of his older brothers Otto III († 1312) and Louis III († 1296).
Stephen was born in Landshut, the son of Henry XIII, Duke of Bavaria and Elizabeth of Hungary. His maternal grandparents were Béla IV of Hungary and Maria Laskarina.
His mother introduced the name Stephen onto the Wittelsbach dynasty with her youngest son. She was a sister of Stephen V of Hungary, and may have named her son after him.
To reduce the influence of the Wittelsbach, Pope Nicholas IV refused his spiritual career in Salzburg and Stephen became a co-regnant of his brothers. During the absence of Otto III in Hungary 1305-1308, Stephen was the only governing duke of Lower Bavaria. Stephen was an enemy of the Habsburgs and died in 1310 during a war against Frederick I of Austria.
In 1299, Stephen married Jutta of Schweidnitz. She was a daughter of Bolko I, Duke of Jawor and Świdnica and Beatrix of Brandenburg. Her maternal grandparents were Otto V, Margrave of Brandenburg-Salzwedel and Jutta of Hennenberg. They had eight children: