The paper declares its editorial position this weekend with a front page designed by Alasdair Gray, the famous artist, author and advocate of a Yes vote.
In its editorial, the Sunday Herald states: ''Scotland is an ancient nation and a modern society. We understand the past, as best we can, and guess at the future. But history is as nothing to the lives of the children being born now, this morning, in the cities, towns and villages of this country.
"On their behalf, we assert a claim to a better, more decent, more just future in which a country's governments will be ruled always by the decisions of its citizens.''
To read the Sunday Herald editorial in full, go here
The paper supported the SNP in the 2007 and 2011 Scottish Parliament elections, but has said it will not automatically favour the SNP or other parties in its news reporting of the Yes campaign during the referendum, and will remain independent and balanced in its reports.
The Herald & Times Group, publisher of the Sunday Herald, The Herald and the Evening Times, is giving the titles' editors freedom to take their own editorial position on the constitution. The company is non-political and neutral.
The Herald has not declared an opinion on the referendum question. It will be up to its editor to decide when and if to do so.
HeraldScotland incorporates content from both print titles and also publishes a balanced range of online-only articles relating to the referendum. Our readers' forum is a neutral commenting facility and our moderating team will remain impartial in the independence debate and will enforce our posting rules rigorously.
Blair Jenkins, Yes Scotland chief executive, said: "The Sunday Herald is the first national newspaper to endorse either side in the debate about our country's future and we are delighted that it has chosen to support a Yes."
He added: "The Sunday Herald's editorial is passionate, inspiring and, above all, a statement of common sense and irresistible logic.
"It is also a very timely intervention following an analysis of the polls this weekend which confirm that support for Yes continues to build while backing for No is sliding backwards.
"What all of this tells us is that more and more people are reaching the conclusion that a Yes vote on September 18 is an opportunity too good to miss.
"With Scotland's future in Scotland's hands we can, should and must build the kind of country that we know Scotland can be."
SNP Westminster leader Angus Robertson said: "This is a very welcome decision by the Sunday Herald and is yet more evidence of the growing momentum behind the Yes campaign.
"The Sunday Herald's editorial gets to the heart of why Scotland can, should and must be independent - the chance to make Scotland a fairer, more progressive country.
"The UK is the fourth most unequal society in the developed world - and with Westminster's welfare cuts and further austerity measures the situation will only get worse.
"With the full powers of independence, we can choose to take a different path and get on with creating the better, fairer, more prosperous country described in the Sunday Herald's eloquent editorial."