The Office for the Protection of the Constitution (BfV) said in its annual report that after suspected manipulation of the US and French elections by Moscow, it was to be expected that Germany would be next.
The report said that German politicians and parties targeted by Russia to spy "on confidential e-mails or other sensitive data must assume that explosive or compromising facts could be made public".
Presenting the findings, Interior Minister Thomas de Maiziere said Berlin was girding for a two-pronged attack of embarrassing leaks and fake news allegedly orchestrated by Moscow.
"We saw a probable influence on the election in America, we saw a probable influence on the election in France," he told reporters.
"There is every reason to believe that that came from Russia and thus it cannot be ruled out that there will be similar attempts on the election in Germany."
BfV chief Hans-Georg Maassen said while he could imagine that Russian President Vladimir Putin "would be happier with another chancellor" than Angela Merkel, the true target of the cyberattacks was Germans' "faith in democracy".
Merkel, widely seen as the European Union's most powerful leader, is seeking a fourth term in the September 24th poll.
De Maiziere noted that Germany's security services had already observed data from computers at the parliament being scooped up, which he said "could be published in the coming weeks".
He said Berlin was bracing for "a classic disinformation campaign with lies and half truths intended to shape opinions" spread by "bots" -- algorithms with fake profiles on social media networks.
"The only thing that helps against that is staying calm, keeping a cool head, finding out the truth and presenting the facts," he said.
Hillary Clinton has said that Russian hacking of her campaign's emails was partly to blame for her defeat in last year's US presidential election to Donald Trump.
And now French President Emmanuel Macron was the victim of a "massive and coordinated" hacking attack blamed on Moscow on the eve of the run-off election.
Russia has steadfastly denied all accusations of state-sponsored cyberattacks.