Its interior minister, Thomas de Maiziere, and spy chief, Hans-Georg Maassen, issued the warning in Berlin on Tuesday (4 July) after unveiling a yearly intelligence report.
De Maiziere said the material “could be published in the coming weeks,” the Reuters news agency reported.
Maassen said Russia’s intention was “to damage trust in and the functioning of our democracy so our government should have domestic political difficulties and not be as free to act in its foreign policy as it is today.
A hacker group, called APT28, said by US intelligence to be a front for Russian spies, stole 16 gigabytes of data from over 5,600 computers in the German parliament in 2015.
It broke into the accounts of 16 MPs and some in chancellor Angela Merkel’s office.
Recalling Russia’s actions in the US and French elections, De Maiziere said he expected "a classic disinformation campaign with lies and half truths intended to shape opinions" that would be spread online by "bots”.
Mainstream parties are polling miles ahead of pro-Kremlin competitors, the Die Linke (The Left) party and the far-right Alternative for Germany (AfD).
The German voting system is also hard to hack. People vote on paper ballots and data is counted on an encrypted, offline network by the Federal Statistics Office.
But Germany hosts three pro-Kremlin media - RT Deutsch, Sputnik Deutsch, and NewsFront Deutsch.
The 2.5 million Russian people living in Germany also follow Russian state media, which has been broadcasting anti-Merkel propaganda for years.
Maassen, the intelligence chief, said on Tuesday: “It [Russia’s campaign] may not be aimed at strengthening one party or another, or ensuring that one or another person is elected to run the government, but that the trust in the functioning of our democracy is damaged.”
The yearly intelligence assessment by Germany’s domestic agency, the BfV, which handles counter-espionage, said Russia, Turkey, and China were its main adversaries.
It said Turkey was spying on Turkish expats in Germany and China was trying to recruit agents on Facebook for economic espionage.
Read more: Germany expects Russian leaking to start in 'weeks'