The move was agreed at the annual conference of the Communication Workers Union, which has 17,000 members working on postal and telecoms services in Scotland.
A motion recommending "all members in Scotland that they vote 'no' in the forthcoming Scottish Independence Referendum" was passed at the 800-strong conference in Bournemouth.
The union's executive recommended a No vote based on the outcome of two branch forums, a series of independent member meetings and two polls of members which showed the majority of Scottish members would vote No (60%).
CWU general secretary Billy Hayes said: "We are recommending a 'no' vote to our Scottish members in September's Scottish Independence Referendum.
"This decision is based on a need for unity against austerity and the barrage of cuts from the coalition Government and the SNP in Scotland.
"This is an extremely important constitutional debate and it is crucial that as a trade union we are able to assist our members on the impact this will have on the Scottish economy and that of the rest of the UK.
"The majority of our members support a 'no' vote as we found when conducting polls and holding forums. A 'no' vote would be in the best interests of our members."
CWU deputy general secretary Andy Kerr said: "We believe that Scottish independence would have a profound effect on the companies we work with. Royal Mail, Post Office Ltd and BT all have extensive networks operating in Scotland and an independent Scotland would put untold pressure on these companies and the jobs of our members.
"It's not clear what the implications would be on Royal Mail's Universal Service Obligation in an independent Scotland, the pension schemes our members are enrolled in or how it would affect the roll-out of broadband services to the inaccessible rural communities in Scotland.
"Independence would have a significant impact on the ability of these companies to invest in their networks."
Commenting on behalf of pro-union campaign group Better Together, Shadow Scottish Secretary Margaret Curran said: "Today the CWU becomes the sixth major trade union to back Scotland's place in the UK.
"A vote for Scotland to remain in the UK is a vote for the trade union way - working together with family, friends and colleagues across the whole of the UK to make Scotland a fairer place to live.
"I am delighted that posties will be working hard to deliver a No vote in September."
Blair Jenkins, chief executive of Yes Scotland, said: "The STUC's (Scottish Trades Union Congress) stated position - and one we believe to be highly significant - is not to support the pro-union campaign backed by Labour, and individual unions representing tens of thousands of workers have already agreed with that line.
"However it should be emphasised that the STUC's position is that it is 'more attracted' to Yes.
"It is our job and intention between now and September 18 to persuade all trade unionists of the opportunities of Yes to build a fairer and more socially just Scotland, to protect and enhance workers' rights and to create new jobs by growing our economy in a responsible and sustainable way."