The programme is part of the Emirates Worldwide Partnership with Rugby World Cup 2015.
Synergy will develop a PR campaign based on Emirates' 'Hello Tomorrow' global brand platform.
Emirates' previous agency was Fast Track.
Adam Raincock, director of comms at Synergy, said: "The prospect of creating a PR campaign for a global brand like Emirates provides an incredible opportunity for the team here to showcase their innovation and creativity. The RWC2015 is the most exciting proposition in this year’s sporting calendar and we can’t wait to get started."
Emirates first sponsored the RWC in France in 2007 as a tournament sponsor, became a worldwide partner for RWC 2011 in New Zealand, and was the first worldwide partner to sign an agreement for both RWC 2015 in England and 2019 in Japan.