The founders of the PR agency, Eddie May and Jim Hawker, will each own 25 per cent, while Blowfish Digital's Farhad Koodoruth will own 50 per cent. The company will still be known as Threepipe.
Alongside PR, the new outfit will include specialists across digital marketing, media planning, SEO and data analytics.
May said the merger followed the two agencies working together on clients, including the Football League.
He added: 'We've been integrating social media and content into our PR campaigns for years, but we saw the need to offer clients a wider range of skills, giving us the ability to create properly integrated and effective campaigns, all under one roof.'
Threepipe's original team of 30 will now work with Blowfish Digital's 20 staff in a new office in Holborn.
The move is the latest example of the industry reacting to the expansion of digital comms. Major developments last year included Bell Pottinger launching Wired and GolinHarris acquiring Fuse.
May pointed to the 'struggle' that the PR industry faced regarding evaluation, adding that digital expertise would help to provide proof of ROI.
He said: 'We think one of the benefits of merging with Blowfish will be that the digital marketing world is a lot further ahead when it comes to showing ROI. While you've had some agencies toying around the edges of digital, we're leaping in with both feet.'
The new agency will combine Blowfish's clients, which include Four Music, eBay and Ted Baker, with those of pre-merger Threepipe. These include Stella Artois, Ericsson and Leapfrog.