Sebastian of Heusenstamm (? - 18 March 1555) was the Archbishop of Mainz from 1545 - 1555.

Sebastian was considered an outstanding scholar in the cathedral of Mainz. At a time when the religious institutions of Germany were in uncertainty, Sebastian became the favourite to succeed the Archbishop Albert III of Brandenburg. In 20 October 1545 the cathedral elected Sebestian, in defiance of the Holy Roman Emperor Charles V who demanded Otto Truchsess of Waldburg, Bishop of Augsburg, be elected. The Catholic princes doubted Sebastian's Catholicism due to his friendship with the Protestant Landgrave Philip I the Magnamious of Hesse, but the cathedral chapter did not doubt it and he was confirmed by the Pope on 2 May 1546.

The first pressing issue of Sebastian was to lead Mainz through the War of the Schmalkaldic League intact. He also had to work with the Counter-Reformation which the Council of Trent was at the same time concerned with, and in 1548 he devised a temporary solution for the Emperor through an interim in which to expand inspection of Catholic priests and bishops and grant some concessions to the Protestants. The Refortmation had greatly divided the archbishopric, so on 19 November 1548 he held a provincial synod where Protestants were invited to reform the catholic church. As a result new liturgy was published, and a policy of peaceful reform (not confrontational) was adopted.

In 1551 he joined the Archbishops of Trier and Colonge at the second sitting of the Council of Trent, and he diplomatically made important suggestions which earnt him the support of the Pope. However in 1552 new hostilities broke out and Sebastian and the other prince-bishops fled back to their lands. Margrave Albert II Alcibiades of Brandenburg-Bayreuth sacked Mainz, and Sebastian soon realised the emperor had not the power to safeguard their lands. So Sebastian organised a peace between surviving prince-bishoprics and prince-abbeys but died in early 1555 before the conclusion of the Peace of Augsburg. He was buried in Mainz cathedral.

Preceded by:

Sebastian of Heusenstamm

Succeeded by:

Albert III of Brandenburg Archbishop of Mainz
1545 - 1555
Daniel Bredel of Homburg