He demanded the moratorium so people in Scotland can have their own say on what do with the public "asset" in the event of a Yes vote next September.
"The Scottish Government is firmly opposed to the privatisation of Royal Mail and this proposed sell-off of an essential public service should be subject to a moratorium until the result of Scotland's referendum is known," he said.
"Privatisation is a deeply flawed decision with potentially serious and long-term effects on Scotland's rural communities and rural economy."
A majority of Scotland's MPs opposed the sell-off in a vote, which he said illustrates that decisions should be taken independently.
"I am therefore calling on the Prime Minister to put a halt to this deeply misguided action until the people of Scotland have had their say on Scotland's future next year," he said.
"The Royal Mail is a profit-making enterprise with massive public assets which are as much the people of Scotland's as they are the rest of the UK's. As a result, it is unacceptable for those assets to be sold off before the people are able to make their decision on Scotland's future.
"The Royal Mail should be in public hands and following independence we will ensure that the people of Scotland have the opportunity to keep it that way. Across Europe there are many successful public postal services. In contrast no country has divided the post office network from the postal service as has been done in the UK as a preparation for sell off.
The UK Government confirmed last Thursday that it is pressing ahead with controversial plans by selling shares, sparking claims it was ''selling off the family silver''.
UK ministers said it was a ''very exciting'' day as details of a share sale were announced, but they were attacked by unions and opposition politicians.
Members of the public will be able to apply for shares - at a minimum of £750 - as well as institutional investors. Analysts expect the sell-off will make up to £3 billion.
Scottish Liberal Democrat leader Willie Rennie said the UK Government's scheme will not affect the universal service.
"What this does is bring more money into the Royal Mail," he told the BBC's Sunday Politics.
"For years under Labour thousands of jobs were lost, many sub post offices closed. We stopped that decline, we've protected sub post offices, we're investing in Royal Mail through this process.
"This will be good for Royal Mail. It will make it flourish."