M&C Saatchi, which has a long relationship with the Conservative Party, will now advise the campaign on its advertising, marketing and message development in the 100 days left before the vote.
Campaign revealed Better Together was talking to agencies last month as part of a last-ditch attempt to shore up support for a "no" vote.
Rob Shorthouse, the director of communications at Better Together, said: "Throughout the pitch process M&C Saatchi demonstrated incredible creativity, but also a passion about, and commitment to, our cause. Like the majority of people in Scotland, they don’t want to see the United Kingdom broken up.
"Over the course of the next 100 days, everyone will be focused on taking our positive ‘best of both worlds’ message to every voter in every part of Scotland. We can have more decisions taken here in Scotland and keep the strength, security and stability of being part of the United Kingdom. We don’t need to choose between the two."
The "Labour isn’t working" ad credited with helping the Conservative Party winning power in 1979 was created by Saatchi & Saatchi, the agency founded by the brothers Charles and Maurice.
The Saatchis left Saatchi & Saatchi and then founded M&C Saatchi in the 1990s. M&C Saatchi worked with the Conservative Party until David Cameron became leader in 2005.
The Conservative Party hired Havas Worldwide ahead of the 2010 General Election but brought M&C Saatchi in with two months to go after a campaign starring Cameron was widely pilloried.
Moray MacLennan, the global chief executive of M&C Saatchi, said: "We have worked on many political campaigns and on many government projects. However, nothing from the past would have as deep and fundamental repercussions for the future.
"The choice that my fellow Scots make on 18 September this year will not just affect people in the here and now, it will impact on the lives of people in Scotland and across the UK for hundreds of years.
"President Obama said that he hopes that the UK remains united. We will be doing everything we can to make sure that it does."