Nikolaj, Copenhagen Contemporary Art Center (Danish: Kunsthallen Nikolaj; previously, Sankt Nicolai Kirke) is two blocks north of Torvet, in Copenhagen, Denmark. Situated within Højbro Plads, it is an exhibition space, especially for contemporary art. Until April 1, 2006, it was known as Nikolaj Udstillingsbygning. The building consists partly of the remains (a tower) of St Nicholas Church, which was destroyed in the fire of 1795.

The original church building was constructed in the early thirteenth century and was Copenhagen's third oldest church.In 1530, the ex-monk Hans Tavsen preached the first Lutheran sermon within Copenhagen in St Nicholas Church. The fire in 1795 burned down most of the building, and from 1805, it was no longer an official church. Though church ruins were demolished, the sturdy tower remains standing in the present day. Butcher stalls occupied the area around the tower until the second half of the 1800s when they were closed. The current building, which opened in 1912, is by a design of the architect, Hans Christian Amberg, representing a modern reconstruction of the destroyed church. The current spire is also a modern reconstruction of the original, financed in 1909 at the initiative and expense of the brewer Carl Jacobsen. He also financed the 1915–1917 repairs.

The first art exhibitions occurred at Nikolaj in 1957. Nikolaj gained prominence after the Fluxus performances of the 1960s.

The tower has served as a naval museum and its attic was at one time a library. It was also the focus of Hans Christian Andersen's drama, Love of Nicolai Tower performed in 1829 at the Royal Theatre. When the internationally notable Danish sculptor Bertel Thorvaldsen returned to Copenhagen in 1838 after living and working for 40 years in Rome, a flag was hoisted on the tower when his ship approached to alert residents of his arrival. The art center's focus is on Danish and international modern art. Within the building there is also a cafe.

here are two annual exhibitions, one by children, and another one by an older artist who is deemed to have had a pioneering effect on modern art. Jananne Al-Ani and Kutluğ Ataman have exhibited at Nikolaj.