Mannesmann was a German industrial conglomerate. It was originally established as a manufacturer of steel pipes in 1890 under the name "Deutsch-Österreichische Mannesmannröhren-Werke AG" (lit. "German-Austrian Mannesmann pipe-works joint-stock company"). In the twentieth century, Mannesmann's product range grew and the company expanded into numerous sectors; starting from various steel products and trading to mechanical and electrical engineering, automotive and telecommunications. From 1955, the conglomerate's management holding with headquarters in Düsseldorf was named Mannesmann AG.

The particular success of the corporate activities in the area of telecommunications that started in 1990 was the predominant reason for the takeover of Mannesmann by the British telecommunications company Vodafone in 2000, still one of the largest-ever company takeovers worldwide. Back then, the Mannesmann Group had 130,860 employees worldwide and revenues of €23.27 billion.

The name Mannesmann ceased to exist in the engineering, automotive and telecommunications sectors soon after Vodafone purchased the company. It lives on in the steel industry, particularly in the steel tube and pipe industry, as the German steel manufacturer Salzgitter AG bought the pipe production division of Mannesmannröhren-Werke AG (today Mannesmannröhren-Werke GmbH), as well as the Mannesmann brand.