Unionskirche, Protestant church in Idstein, Germany.

The Unionskirche is a Protestant parish church on Martin-Luther-Straße in Idstein's old town. Built from 1330 to 1350 as the Gothic basilica of St. Martin, it was rebuilt from 1665 to 1677 into a Baroque sermon and court church with marble arcades and three-sided galleries. Its nave ceiling and upper walls are covered by 38 oil paintings on biblical themes.

A Romanesque church stood on the site before 1287, evidenced by remains in the tower north of the choir. The present Gothic basilica of St. Martin was built from 1330 to 1350 under Count Gerlach of Nassau for a college of six canons founded in 1333. After the Reformation, Count Johann of Nassau-Idstein rebuilt the interior from 1665 to 1677 as a Baroque sermon and court church; it was renamed Unionskirche in 1917 for the 1817 Nassau Union.

The name Unionskirche, adopted in 1917, marks the 1817 Nassau Union of Lutheran and Reformed churches, which unlike Prussia's was a free decision of the pastors. The parish choir, the Idsteiner Kantorei, has given two concerts a year since 1972, and cantor Carsten Koch began a Beethoven symphony cycle on the Day of the Open Monument in 2004, ending with the Ninth on 9 September 2012.

The church sits in Idstein's pedestrian area and is easily reached on foot, especially when walking through the old town's narrow streets. Visitors should note that access may vary depending on events taking place, so checking ahead before a visit is recommended.

The 38 ceiling and wall paintings were made from 1673 to 1678 by Antwerp painter Michael Angelo Immenraedt and his assistant Johannes Melchior Bencard. Three designs came from Joachim von Sandrart and five were executed by his nephew Johann von Sandrart; several compositions draw on works by Rubens.