Proof co-founder Richard Moss and directors Jim Sutton and Joe Meaney will remain in place as the Proof leadership team and will report to April-Six CEO Fiona Shepherd.
Sutton would not disclose the value of the acquisition but told PRWeek that he felt it would help the agency, which currently employs 14 staff, to "grow faster".
Moss added: "Over the past decade, we have grown Proof into one of the most respected technology, science and engineering communication companies in Europe.
"April-Six’s results-led approach fits perfectly with our constant drive to demonstrate the tangible impact of public relations. By combining our existing services we can meet the growing demand for more innovative and integrated communication programmes."
Since launching in 2004, Proof has worked with clients including Cyber Security Challenge, CompTIA, Royal Academy of Engineering and National Physical Laboratory.
As part of the acquisition its South African offshoot, Proof Communication Africa, has now been demerged and will be taken forward separately by Proof co-founder Jonathon Rees.