Iraqi Prime Minister Nouri Maliki has suspicions about corruption within his own team, his spokesman told the BBC.
The purchase - said to include attack helicopters and missiles - was only signed off in October.
Iraq has been rebuilding its armed forces since the end of US-led combat operations against insurgents.
One Russian military expert has suggested that the Iraqi authorities scuppered the Russian arms deal under pressure from Washington.
Announcing the cancellation of the purchase on Saturday, a spokesman for Prime Minister Nouri Maliki told AFP news agency that "when Maliki returned from his trip to Russia, he had some suspicions of corruption, so he decided to review the whole deal".
"There is an investigation going on, on this," he added.
The sale would have made Moscow - the main supplier of arms to Iraq under Saddam Hussein - the country's second-biggest arms supplier after the US.
There has been no word from Russia about the cancellations.
In early October, Mr Maliki said in a speech that he did not want Iraq to be "part of someone else's (arms) monopoly."
But he faced criticism from political opponents who questioned buying from Russia, when multiple deals with the US had been signed.
One Iraqi MP suggested that counterterrorism operations - the stated aim of the purchase - required improved intelligence, and not the 30 Mi-28 attack helicopters that were reported to be part of the deal.
The contracts were announced to some fanfare on 9 October after talks between the two countries' prime ministers near Moscow.
Mr Maliki - who said he was seeking "quick contracts to fight terrorism" - warned even before he left that anything he signed might be scuttled by parliament.
And at the time the deal was agreed, analysts suggested that while it was clear Iraq wanted to diversify its weapons purchases, buying from Russia would only encourage the sense in Washington that the US was somehow "losing Iraq".
Igor Korotchenko, head of the Moscow-based Centre for Analysis of World Arms Trade, told the BBC the cancellation was "absolutely unprecedented in the history of the Russian arms trade".
"As soon as the deal was announced a month ago I said that the US would not allow Iraq to buy such huge quantities of weapons from Russia. I believe Washington regarded this as an absolutely unacceptable scenario," he said.
"As far as talk about corruption is concerned, I think it's a smokescreen," he said, adding: "I can't see any scope for corruption in the Iraq deal. I believe this is just a pretext and the true reason is Washington applying pressure on Baghdad."