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.
"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."
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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 24 poll.
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.
Russia has steadfastly denied all accusations of state- sponsored cyberattacks.