ThyssenKrupp said technical trade secrets were stolen from the steel production and manufacturing plant design division, as well as other areas yet to be determined.
The attackers were based in Southeast Asia, according to ThyssenKrupp, and had engaged its database with “organised, highly professional hacker activities”.
The cyber attack was discovered by the company’s internal security team in April and traced back to February.
ThyssenKrupp's disclosure follows an attack last week on routers owned by German telecoms giant Deutche Telekom, that caused an outage for nearly 1m customers.
German press reported the security breach targeted ThyssenKrupp’s Hagen Hohenlimburg specialty steel plant in western Germany, though the company declined to identify which locations were infected or the scale of the intellectual property losses.
Yesterday, Tata unveiled a rescue deal for its Port Talbot plant and UK strips product business, agreed to with unions, though did not provide an update about the ThyssenKrupp talks to launch a possible joint venture.