The agency won a competitive pitch to work on the two-day event, part of a larger global series of rugby sevens events featuring international teams.
Frank PR will help raise the profile of the new Feast of Rugby format to target new audiences and drive ticket sales.
This new concept will see the stadium transformed into a "festival of rugby and food", with local and artisan food vendors.
Nic Fletcher, head of marketing at the RFU, said: "PR plays a huge part of our integrated marketing campaign for the HSBC London Sevens so it’s essential that we work with an agency that has strong sport and event experience. The exciting changes to this year’s theme also reinforce the need to tell this story successfully and creatively, and we have confidence that Frank PR has the ability to do this."
Rugby sevens will be an Olympic sport for the first time in Rio 2016, having landed a place on the Olympic roster in 2009 and then embarked on a PR drive through the agency Braben.