Propeller was appointed without a competitive pitch.
SapientNitro has annual billings of more than £165.4m and works with brands including British Airways, Marks and Spencer, Royal Bank of Scotland/NatWest, Unilever and Vodafone.
Craig Smith, European marketing director at SapientNitro, said: "As a ‘new breed’ of agency dedicated to making an impact for our clients in, and across, brand communications, digital engagement and commerce, SapientNitro needs a PR partner that understands the enormity and complexity of the convergence taking place. With its proven track record in delivering specialist and national coverage, Propeller PR is a perfect partner for our next phase of growth."
Martin Loat, CEO at Propeller Group, said: "SapientNitro is one of the great success stories in digital media, helping the world’s best known brands to evolve in the digital age."
Publicis’ acquisition of Sapient, which took many in the industry by surprise, was part of as a move to shift the company’s focus towards digital, aiding its plan to derive half of revenues from digital and technology three years ahead of its 2018 plan. It reached this landmark in the first quarter of 2015.
Speaking to PRWeek after agreeing to buy the business in November 2014, Publicis CEO Maurice Levy said he wanted a "cross-pollination" of all the services offered by the group for clients of Publicis and Sapient.
He added: "Clients will have access to new tools and possibilities for communications. We are in the people business and broadening the base of collaboration opens up new horizons and opportunities."