Troisdorf (German pronunciation: [ˈtʁoːsdɔʁf] ⓘ) is a city in the Rhein-Sieg-Kreis (district), in North Rhine-Westphalia, Germany.

In 1969, the urban area expanded with the annexation of the township of Sieglar and the villages of Altenrath and Friedrich-Wilhelms-Hütte (total population in 1969: about 51,000).

On June 4, 1972, Troisdorf founded the first advisory council for foreign citizens in Germany.

In the years following the Peaceful Revolution and German reunification of 1989/1990 many migrants from Russia and other East European countries settled in Troisdorf.

Troisdorf is one of the German cities whose mosque includes a minaret; it was built for the local Islamic community.