The First Minister, in the unexpected announcement to Holyrood yesterday, said he would bring the postal service "back into public ownership" north of the Border if the UK's Government proposed sell-off goes ahead.
The commitment came a day after Finance Secretary John Swinney said it was "impossible to give a definitive answer" about the prospects for renationalising the Royal Mail.
Pressed to explain the new pledge, officials said they could not estimate the cost of buying back those parts of the Royal Mail based in Scotland.
However the First Minister's chief spin doctor said existing universal obligations, including six-day deliveries and standard stamp prices across the country, would be maintained.
He insisted the announcement had been planned but was unable to say whether it had been agreed by Cabinet.
The First Minister made the promise as he was quizzed during First Minister's Questions.
Asked by Tory MSP Gavin Brown, the Scottish Conservatives' finance spokesman, if he was making a clear commitment to renationalising the Royal Mail, Mr Salmond replied: "An independent Scottish Government that I lead, because it will be a decision of the Scottish people, will bring the Royal Mail, our postal service, back into public ownership."
He said his first priority was to block the sale or at least delay it until after the independence referendum, and added: "The issue gets to the nub of why Scotland needs independence."
Mr Salmond had called for the sell-off to be put on hold until after the referendum on September 18, 2014. He set out his demand in a letter to the Prime Minister David Cameron yesterday.
Responding to Mr Salmond's announcement, Mr Brown said: "I am not sure who was more astonished by the First Minister's answer - me or members of the Scottish Cabinet.
"I am calling on them to publish their costings, in full, as soon as possible."
Labour MP Ian Murray, who is leading the party's campaign against the privatisation, said: "This is yet another off the cuff, uncosted promise by Alex Salmond.
"Given the Scottish Government has had no discussions with either the Royal Mail or UK Ministers and the fact the geographical nature of Scotland makes postal services the most expensive to maintain in the UK, the First Minister must tell us how much it will cost.
"The SNP have committed to maintaining the six day a week, one price goes anywhere service should Scotland become independent so the First Minister must tell us how much of the historic Royal Mail pension deficit he will take on, how many staff the Royal Mail will employ and, simply, how much will a stamp cost?"
Scottish LibDem leader Willie Rennie said taking the Royal Mail back into public ownership would break up a UK-wide service.
He warned: "Alex Salmond should explain this reckless move to the thousands of people across the UK who rely on a UK-wide postal service."
The Royal Mail sell-off is expected to raise £3 billion when shares are floated.
The Coalition Government's Business Secretary Vince Cable says it is necessary to secure a "healthy future" for the service.
It is opposed by the Communications Workers Union, which is balloting members over strike action. The result of the ballot will be announced in October.