“There has been an attack on the IT systems of the Bundestag,” spokesman Ernst Habeker said in Berlin.
Experts from the Bundestag administration and the Government Office for Information Technology Security (BSI) are working to fend off the hackers, he added.
Spiegel Online reported that parliament IT specialists noticed several days ago that someone was trying to gain access to the Bundestag's internal network in a “serious” attack.
It is not yet known whether any computers containing sensitive information were penetrated.
MPs and their assistants from several parties were warned about the attack on Friday morning, and were initially told that the network would be shut down in the afternoon.
While a full shutdown didn't turn out to be necessary, some computers were switched off – including machines containing information that originated at the inquiry into spying by the US National Security Agency (NSA) in Germany.
The last time the Bundestag and Chancellery came under attack from hackers was in January, leaving both institutions paralysed for several hours.
At the time, a pro-Russian hacker group in Ukraine claimed responsibility.