Welf I (died about 9 November 1101, Paphos) was duke of Bavaria from 1070 to 1077 and from 1096 to his death. He was the first member of the Welf branch of the House of Este. In the Welf genealogy he is counted as Welf IV.

Welf was the son of Azzo II of Este and his wife Chuniza of Altdorf. When Welf's maternal uncle, Welf, Duke of Carinthia (also known as Welf III), died childless, Welf inherited his property.

Welf married Ethelinde, daughter of Otto II, Duke of Bavaria. When Duke Otto had become an enemy of King Henry IV, Welf divorced Ethelinde, and soon thereafter (in 1070) was appointed duke of Bavaria in Otto's stead. During the Investiture Controversy, Welf sided with Pope Gregory VII, and in March 1077 supported the election of Rudolf of Rheinfelden as anti-king. In May 1077, he was banned by the king.

In 1089, Welf's son Welf married Matilda of Tuscany, thus strengthening relationships with the pope. After the younger Welf divorced Matilda in 1095, Welf made amends with King Henry IV and was reappointed as duke of Bavaria.

After the death of his father Azzo in 1097, Welf tried to acquire his father's property south of the Alps, but did not succeed against his younger half-brother Fulco.

In 1099, Welf joined the Crusade of 1101. He died while returning from the crusade in Cyprus in 1101 and was buried in Weingarten Abbey. He was succeeded as duke of Bavaria by his son Welf.

Welf had the following children from his marriage with Judith of Flanders, daughter of Baldwin IV, Count of Flanders:
• Welf, born 1072
• Henry
• Kunizza, died 6 March 1120, married Frederick Rocho, Count of Dießen