Stadttheater Klagenfurt, Opera house and theatre in Klagenfurt, Austria.

Stadttheater Klagenfurt is a municipal theatre built in 1908–1910 by the Viennese office Fellner & Helmer, a near-identical copy of their theatres in Gießen and Gablonz. Its classical facade with Art Nouveau interiors carries the inscription "JUBILÄUMS STADT THEATER", the Carinthian and Klagenfurt coats of arms, and the years 1908 and 1910. The auditorium has tiered beige seats, two curved balconies, and a vaulted ceiling with recessed panels.

The first theatre on the site was a Ballhaus built between 1605 and 1620 for the aristocracy, where Italian troupes stopped on their way from Venice to Vienna. It opened as a theatre in 1737 under the Carinthian estates, and the wooden structure was rebuilt in stone in 1811 as the Altes Theater. The present house by Fellner & Helmer was begun in 1908 and opened on 22 September 1910 as the Kaiser Franz Josef I. Jubiläumstheater, a near-identical copy of their theatres in Gießen and Gablonz.

The venue hosts opera, drama, and musical performances throughout the year and serves as a gathering place for theater enthusiasts from the region. The productions shape the city's cultural life and draw audiences from near and far.

The theater offers smartphone tickets for convenient entry to performances. Young people under 27 and children can attend shows free of charge through a partnership with Volksbank Kärnten.

The theatre's interior was rebuilt from 1996 to 1998 to plans by Günther Domenig, who added a modern wing at the rear of the 1910 Fellner & Helmer house. The building is a near-identical copy of the municipal theatres in Gießen and Gablonz, built in 1906 and 1907.