Religion

Due to the Nassau prince's Christian influence, most of Siegen's religion is of the Christian religion, divided between Roman Catholic and Protestant. The town had originally belonged to the Archbishop of Mainze, and it had a White Nun convent before it was ruined by a fire in the 15th century. A Franciscan monastery was also built in Siegen before its dissoution in 1533, when Nassau overlords introduced the Reformation.

Siegen was of the Lutheran church until 1950, when the principality of Nassau converted it to the Reformed church. While Siegen was a predominantly protestant town, it was not ingrained enough to withstand a counter reformation in 1623, where it succeeded to convert a fifth of the population back to Catholicism. 3 years after that, the Jesuit monastery was formed.

In 1815, during the Prussian rule, the Lutheran and Reformed churches were united, copying the model of the Prussian church, though it should be noted that town parishes still kept their reformed imprint. Due to its part as a Westphalia provincial church, Siegan then became the seat of a Superintendency, which has a similar modern entity in the form of the Kirchenkreis, or church district, where all parishes belong, barring Free Church parishes.

To date, Siegen's religion is still predominantly divided between the Catholics and the Protestants, though other forms of Christianity also blossomed. There is the Greek Orthodox, the Romanian Orthodox and Baptists, the Evangelical-Methodist Church, the Christian Assembly, and the Seventh-day Adventist Church.

Other radical religious communities include the Church of Jesus Christ of Latter-Day Saints, the new apostolic Chuch, and Jehova's witnesses.

Non-Christian religions include the Bahai and Islam, due to the big number of Turkish and Arabic migrants in Siegen. Today, there are more than 15,000 Muslims in Siegen.