Spiegel Online reported that employees in the lower chamber of parliament have had no Internet access since around 1400 GMT.
"The last email in the inbox came in at 14:24," a lawmaker's aide told Spiegel.
Die Welt daily said investigators were examining if there had been a hardware fault, or if the network had been hit by hackers.
The Bundestag's press service was not immediately available for comment.
The German parliament in 2015 fell victim to an attack that security services have since blamed on Russia.
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In the run-up to general elections in September, authorities have repeatedly warned that Germany could see more cyberattacks.
Chancellor Angela Merkel has also warned that cyberattacks from Russia are now so common that Germany must learn to cope with them as part of daily life.
In September, several political parties were sent fake emails purporting to be from NATO headquarters but which in fact contained a link that installed spying software on victims' computers.