Theater am Marientor, Musical theater building at Plessingstraße, Duisburg, Germany.

Theater am Marientor is a theatre building in Duisburg, erected in 1995 to plans by architect Helmut Kohl. Its auditorium holds 1,524 seats, and a room-acoustics control system installed later lets the hall meet concert-hall requirements from chamber to philharmonic programmes. The city made the plot available cheaply for the construction.

The Theater am Marientor was built in 1995 to plans by architect Helmut Kohl, and the Stella AG used it as the stage for its production of Les Misérables from January 1996 to November 1999. When the operator went insolvent, the musical closed in 1999 and a city property subsidiary bought the building back. It then served as the Duisburger Philharmoniker's substitute venue until the new Mercatorhalle opened on 21 April 2007.

The Theater am Marientor draws audiences from across the Ruhr region, not just from Duisburg itself. The program tends to focus on popular musicals and family-friendly productions, which gives the venue a welcoming and open character that sets it apart from classical opera houses.

The theater is within walking distance of Duisburg's city center and easy to reach by public transport. On performance evenings, the area around the entrance gets busy, so arriving a little early makes entry more comfortable.

The theater has been used as a filming location for German television productions, including casting shows and comedy programs. For some visitors, the building feels familiar before they even step inside, because they have already seen it on screen.