Siegfried I (? - 16 February 1084) was the Abbot of Fulda from 1058 until 1060, and the Archbishop of Mainz from 1060 until 1084.

Siegfried was born to the Franconian aristocratic family of the Reginbodonen. He was close friends with the counts of Königssondergau, the Burgraves and Archstewards of Mainz and the High Stewards of Fulda. Siegfried was educated in Fulda and there became a monk. On 25 December 1058 he was appointed its Abbot. On 6 January 1060 the Empress Agnes appointed him the Archbishop of Mainz. In 1062 Siegfried joined the group led by Anno II, Archbishop of Cologne which captured the young King Henry and the Imperial Insignia. Siegfried spent the rest of his life in the influence of Anno or Adalbert I of Saxony, Archbishop of Bremen.

During the winter of 1064/5 Siegfried undertook a pilgrimage to Jerusalem. In 1070 he went to Rome to voluntarily retire from the Archbishopric, however Pope Alexander II forbade it. Together with Anno II, he coverted the Abbey of Saalfeld to the Benedictine Order. In 1072 he met Hugh, Abbot of Cluny, and took the meeting as an excuse to undertake the pilgrimage to the Santiago de Compostella. Since then Siegfried was an advocate of the Clunian Reforms, which resulted in the founding of the Abbeys of Ravengiersburg and Hasungen in 1074.

With the outbreak of the Investiture Controversy between the Pope and Emperor, Siegfried was at first clearly a partisan of King Henry IV. However at Trebur in 1076 he used the opportunity to switch sides and supported Pope Gregory VII. He was quickly expelled from Mainz by the royalist citizens and spent the remainder of his life in Hasungen. He advocated war against Henry IV with the crowning of anti-kings, and he crowned Rudolph of Rheinfelden in 1077 and Herman of Luxembourg in 1081. From 1081 he was inactive politically. He died in 1084 and was buried in Hasungen.

Preceded by:

Siegfried I

Succeeded by:

Leopold I Archbishop of Mainz
1060 - 1084
Egbert Abbot of Fulda
1058 - 1060
Widerad of Eppstein