Hackers believed to be from Southeast Asia were trying to obtain "technological know-how and research results" from the steel conglomerate, said a company spokesman, confirming a report in the Wirschaftswoche weekly.
"The attack is over and had been repelled," he added.
The "massive cyber attack" had targeted divisions dealing with planning of industrial plants and steel works in Europe.
Highly protected parts of the company such as ThyssenKrupp Marine Systems or the IT control systems of the group's blast furnaces and power plants were not affected.
ThyssenKrupp Marine builds warships including submarines for the German and Israeli navies.
The cyber attack was uncovered by the company's IT security office, which monitored and analysed the hacking while it was ongoing, said the group.
The company said it was unable to estimate if there had been any material losses arising from the attack, adding that only "data fragments" had been stolen.
There was no sign of data manipulation or sabotage, it added.
Germany has been the target of repeated cyber attacks in recent years.
Internet services for a million households went down last month after Deutsche Telekom was hit by hackers.
Berlin has also ramped up its warning of a rise in cyber attacks orchestrated by Russia aimed at disrupting next year's general election.
Chancellor Angela Merkel said such assaults are now so common that Germany must learn to cope with them as part of daily life.