Upon the commissioning of the first subway section of the U1 from Reumannplatz on February 25, 1978, Karlsplatz also became an underground station and, after the platforms on Line U4 opened in 1980, is now the largest transport hub for Wiener Linien.

The station is located in the city center and is surrounded by many important buildings and other landmarks: The exit via Opernpassage leads to the main building of the Vienna University of Technology, Resselpark and the Karlskirche, while the Northern exit leads to Vienna State Opera, and the entrance to the pedestrian zone Kärtner Straße.

Via the underground pathways you can also reach Secession Building, Naschmarkt, Vianna Künstlerhaus and Wiener Musikverein easily by foot.

In the early railway project plan from 1890, the station can be found under the name Schwarzenbergbrücke (Bridge of Schwarzenberg).

The architecture of the station, which was constructed under the Commission für Verkehrsanlagen in Wien and completed in March of 1898, was unique for the railway network: On the overground there are two pavillons, decorated with ornaments.