Gerard I of Dhaun (? - 25 September 1259) was the Archbishop of Mainz from 1251 until 1259.

Gerard was a member of the house of the Wildgraves of Dhaun and Kyrburg. Gerard had a career in the cathedral of Mainz before he left it in 1244 after he refused to follow the anti-Hohenstaufen policy of the then archbishop Siegfried III of Eppstein. In 1249 he reconciled with Siegfried and joined him. After Christian II of Bolanden was paid to leave the archbishopric by Pope Innocent IV, Gerard was elected the new Archbishop of Mainz. Once elected, however, Gerard took a pro-Hohenstaufen policy and supported King Conrad IV.

In February 1254 he concluded an eternal peace with the Bishopric of Worms, and in May at Bingen it was expanded to include the Archbishoprics of Trier and Cologne and the Bishopric of Basel, forming the beginning of the Rhenish League. But the alliance broke up after the death of King William II of Holland in battle, beginning a period of instability known as the interregnum in which traditional rivalries between the archbishoprics subsided and a lack of central authority promoted sovereignty of the German states. In 1255 he was forced to grant the city of Erfurt several rights. Gerard quickly became embroiled in a dispute with Duke Albert the Great of Brunswick-Brunswick in 1256.

At the election of 1257 the six electors votes were split between Richard, the Duke of Cornwall, and Alphonse X of Castile, and none of the electors were willing to grant the King of Bohemia the deciding vote. Gerard's attempts to use the interregnum and warring between the factions to his own advantage failed, and he prevented from annexing territories in Hesse. Gerard died in Erfurt in 1259 and was buried in its cathedral.

Preceded by:

Gerard I of Dhaun

Succeeded by:

Christian II of Bolanden Archbishop of Mainz
1251 - 1259
Werner of Eppstein