The Foreign Ministry said helping arm Georgia would be "extremely dangerous" and would amount to "nothing but the encouragement of the aggressor."
The warning comes days after Georgian President Mikhail Saakashvili stridently vowed to rebuild and prepare his armed forces for missions other than peacekeeping - comments made alongside a top U.S. general.
It also follows Wednesday's first meeting between Presidents Barack Obama and Dmitry Medvedev, where the two leaders tried to mend serious differences that have plagued ties in recent years - including over last year's war in Georgia.
The brief war helped send relations between Moscow and Washington to Cold War lows, as the Russians routed the Georgian army and humiliated Saakashvili.
The Georgian leader has since repeatedly vowed to rebuild his forces. On Monday, Saakashvili told visiting Gen. James Cartwright, vice chairman of the U.S. joint chiefs of staff, that the country would build "modern, significantly higher quality, significantly stronger armed forces."
The forces "have been trained for peacekeeping operations. Now we are preparing qualitatively different armed forces. Let no one have any illusions," he told Cartwright.
The meeting prompted two days of harsh reports on Russian state-run TV, with one anchor calling Saakashvili's announcements "strange," among other things.
The war "proved that it would be extremely dangerous for neighbors, the entire region and Georgia itself to arm the incumbent Georgian administration," ministry spokesman Andrei Nesterenko said.
"Hopefully, the United States, Ukraine and other countries, which regard themselves as responsible international participants, will take that into account in their policies."
Washington found a staunch ally in Saakashvili, giving military and economic aid to Georgia for years, sending military instructors to train Georgian troops and backing its bid to join NATO.
That has worried Moscow, which considers Georgia part of its historical sphere of influence and which fears a greater U.S. presence in the former Soviet Caucasus or Central Asia.
At their first meeting in London Wednesday, Obama and Medvedev discussed the situation in the South Caucasus. U.S. officials said Obama told Medvedev that the separatist regions of South Ossetia and Abkhazia - both of which are backed by Russia - would never be recognized as independent by the United States.
Russian troops have remained in part of Abkhazia and Georgia, and Moscow plans on building bases there.
On Thursday, meanwhile, Saakashvili welcomed a visiting U.S. missile frigate - the USS Klakring - to the port of Batumi, saying he had been assured that the Obama administration fully supported him.
"Georgia needs strong allies. Georgia needs the hands of friends for the resolution of further problems," he said. "In this matter, in particular after yesterday's meeting in London, I can practically exclude any Russian military adventures in the future - and Georgia will not be happy until it is free of occupation."