Gazprom said stopping another turbine at the Nord Stream 1 pipeline would cut daily gas production to 20 percent, halving the current level of supply.
The German government said there was no technical reason to limit gas supply.
It is likely to make it more difficult for European countries to replenish their stores of gas before winter.
The Nord Stream 1 pipeline, which pumps gas from Russia to Germany, has been running well below capacity for weeks and was completely shut down for a 10-day maintenance break earlier this month.
Russia supplied Europe with 40 percent of its gas last year, and the EU has accused Russia of using energy as a weapon.
The European Commission has urged countries to cut gas use by 15 percent over the next seven months after Russia warned it could curb or halt supplies altogether.
Wholesale gas prices have soared since Russia invaded Ukraine in February, with a knock-on impact on consumer energy bills.
Russian state-controlled Gazprom said the latest reduction in supply would begin at 4am GMT on Wednesday due to the “technical condition” of one of the last two operating turbines.
But a German economy ministry spokeswoman told AFP news agency: “According to the information we have there is no technical reason for a reduction of deliveries.”
The Kremlin maintains that it is a reliable energy partner, and blames Western sanctions for the recent disruption of gas supplies to Europe.
Gazprom said the delayed return – because of sanctions – of equipment serviced in Canada has forced it to keep the gas flow through Nord Stream 1 to just 40 percent of capacity.
“Our product, our rules. We don’t play by rules we didn’t create,” Gazprom chief executive Alexei Miller has said.
The continued reduction in gas supply through Nord Stream 1 is likely to make it more difficult for countries to replenish their stores before winter, when gas usage is much higher.
Gazprom has cut gas supplies altogether to Bulgaria, Denmark, Finland, the Netherlands and Poland over their refusal to comply with a Kremlin order to pay their bills in roubles, instead of euros or dollars.