Siegfried I (died 16 February 1084) was the Abbot of Fulda from 25 December 1058 until 6 January 1060, and from January 1060 until his death in February 1084, he was Archbishop of Mainz.
Siegfried was a member of the Frankish Reginbodonen family of the Rhineland. His father, also called Siegfried, was count of the Königssondergau. Count Siegfried was succeeded by his son Udalric, who was count of the Königssondergau and advocate of the diocesan church of Mainz from 1052 to 1074.
Siegfried was educated in the monastery of Fulda and became a monk there. On 25 December 1058, he was appointed abbot of Fulda and on 6 January 1060, the Empress Agnes appointed him Archbishop of Mainz. In Spring 1062, he entered the political realm as a member of the faction surrounding Anno II of Cologne, who forcibly took control of the regency of the young king, Henry IV in the Coup of Kaiserswerth. Nevertheless, Siegfried never had the political influence of Anno or Adalbert of Bremen, and remained a 'third force'.
In Winter 1064 – 1065, he undertook a pilgrimage to Jerusalem. In 1069 he presided over the assembly of Worms, at which Henry IV announced his intention to repudiate his wife Bertha. Siegfried wrote to Pope Alexander II asking for help with the matter. In 1070, he took a pilgrimage to Rome to seek the permission of Pope Alexander II to lay down his title and abdicate, but the pope refused him. Together with Anno II of Cologne, in 1071, he founded a Benedictine monastery at Saalfeld.