Kapfenberg ([ˈkapfənbɛʁk]), with 22,182 inhabitants in 2023, is the third largest city in Styria, Austria, and lies in the valley of the Mürz near the town of Bruck an der Mur.

Every year in June, the Ritterfest ("knights' festival") takes place at the castle, which is one of the largest events of its kind in Austria.

Kapfenbergs main employer is the steel manufacturer voestalpine.

In 1970, Kapfenberg hosted the European Team Chess Championship, which was won by the Soviet Union.

The highest point is the Floning at 1583 metres above sea level, the lowest point at 483 metres above sea level is the Mürz on the municipal border with Bruck an der Mur, which flows into the Mur around 1.2 kilometres downstream.