The Unionskirche (Union Church) is a protestant parish church in the historic centre of Idstein. The church was built from 1330 to 1350 under Gerlach, count of Nassau, when it was founded as collegiate Church of St. Martin. In 1669, it was transformed into a Lutheran sermon and burial church for the counts and princes of Nassau.

The town church became its current name “Union Church” in 1917, hundred years after the union of Lutheran and Reformed Protestants in the Duchy of Nassau, which was the first in Germany.

The exterior of the church is simple, but it has an extraordinary interior.
The most prominent decorations in the church are the 38 paintings by the Flemish baroque painter Michael Angelo Immenraedt and his assistant. These canvas oil paintings on the ceiling of the nave and the upper part of the walls show biblical scenes and were made from 1673 to 1678. Opulent use of marble from for the arcades, columns, altar, font, pulpit and tombs is another feature of the church. The altar (1676) portrays the “Last Supper” in form of a painting. The crystal chandeliers - originally used in the Kurhaus of Wiesbaden - were fitted in the church in the 19th century.