"The finance ministry considers the ruble extremely undervalued and is starting to sell its left-over currency on the market," spokeswoman Svetlana Nikitina said. She did not give details of the amount.
The ruble rose in value after the announcement by the ministry to 81.61 to the euro and 65.15 rubles per dollar.
"We'll do it for as long as it is needed," deputy finance minister Alexei Moiseyev was quoted as saying by Interfax news agency.
The finance ministry said it has around USD 7 billion at its disposal to prop up the currency. It was not going to crack open a rainy day fund.
At an emergency meeting yesterday the Russian government came up with a list of measures to stabilise the situation, economy minister Alexei Ulyukayev said.