Following the deal, Definition's London team will move into Redhouse's Clerkenwell office. Redhouse managing director Fiona Corey will join the group board.
The acquisition follows that of employee engagement agency Words & Pictures earlier in November, and of Limelight in 2018.
Definition, which has offices in London, Leeds and Dubai, said the Redhouse deal "enhances [our] ability to drive brand creation, recognition, engagement and growth for clients, through seamlessly integrated internal and external communications".
Redhouse has a full-time team of seven and generates annual revenue of about £700,000. Its substantial public sector client list includes HS2, HM Parliament, UK Export Finance, the Foreign and Commonwealth Office, and the Environment Agency. Corporate clients include Pearson, the Institute of Physics, The Institute of Asset Management and EDF Energy.
The acquisition moves the combined group's annual revenue to about £3.8m, with 40 employees.
Definition chairman Nigel Howes said: “We have a bold and ambitious growth strategy based on a shared vision of the future. Our aim is to deliver significant value to our clients by acquiring and leveraging a range of complementary skills and services.
“Redhouse is a very good strategic fit because branding sits at the heart of everything we already do in reputation management at Definition and employee engagement at Words & Pictures. Its expertise extends our sector specialisms into government and, through major departmental contracts, also strengthens our credentials in the transportation and education sectors.”
Corey said: “For over 30 years, blue-chip companies, government departments and public sector organisations have called on Redhouse to create meaningful connections with their internal and external audiences. The ability to blend the group’s PR, external reputation management and internal communications services into our offer through a ‘one-team’ approach will not only be of major benefit to our clients but also offer fantastic new career opportunities for our people.”