Siegfried III of Eppstein (c. 1194 - 9 March 1249) was the Archbishop of Mainz from 1230 until 1249.

Siegfried was a nephew of Siegfried II of Eppstein, Archbishop of Mainz, and also Theodoric II of Wied, Archbishop of Trier, through his mother's side. In 1220 he became a member of the cathedral in Mainz, provost of St Bartholomew in Frankfurt, and provost of St Peter and Alexander in Aschaffenburg. In October of November he was elected the archbishop of Mainz in succession to his uncle. As Archbishop Siegfried was a skilled politian and a supporter of the Hohenstaufens, but he was also like all other Imperial princes through the Confoederatio cum principibus ecclesiasticis of 1220 and he expanded the size of the secular territory.

In 1232 the Holy Roman Emperor Frederick II Stupor Mundi granted the Archbishopric the rich Imperial Abbey of Lorsch. In 1234 Henry, Frederick's son, revolted and attempted to gain Siegfried as an ally. In 1235 Frederick returned and the revolt died away, and Henry was dethroned and imprisoned in Apulia. In August 1235 Frederick and Siegfried held a grand Imperial Diet in which the Peace of Mainz was decreed. In 1237 he crowned Frederick's next son Conrad as King, and Siegfried was granted the guardianship over Germany.

Soon after a new conflict broke out between Frederick and Pope Gregory IX and Frederick was twice excommunicated in 1239. On 24 April 1240 Gregory IX also excommunicated Siegfried who refused to turn against the Emperor. A large war might have broken out but the arrival of the Mongol hordes on the empires eastern border quelled all thought of internal war in the Empire. However in 1241 after Gregory's death Siegfried allied with Conrad of Hochstaden, Archbishop of Cologne, against Frederick and announced intentions that Landgrave Henry Raspe of Thuringia succeed him. Frederick immediately stripped Siegfried of his governorship over Germany, and as Pope Celestine IV had died after only 17 days followed by a two year Papal vacancy, there was no pope to mediate the dispute.

In 1243 Innocent IV became Pope, and he immediately worked against the Emperor. Siegfried was made a papal legate with power over Germany. In 1244 to win over the citizens of Mainz, he gave it far-sweeping privileges which made it an Free and Imperial City, and he also finally managed to win over Henry Raspe. He crowned him anti-King in 1246. Henry Raspe's early death in 1247 proved only a minor setback as he and the other Rhenish archbishops elected Count William II of Holland as new anti-King. Therefore Siegfried was heavily involved in instrumenting the fall of the Hohenstaufens, but he died in Bingen in 1249 before it occurred. Siegfried was buried in Mainz Cathedral.

Preceded by:

Siegfried III of Eppstein

Succeeded by:

Siegfried II of Eppstein Archbishop of Mainz
1230 - 1249
Christian II of Bolanden