John Brown's portfolio of clients includes print and digital publishing work for Chelsea FC, John Lewis, Royal Bank of Scotland and Waitrose.
Established in 1987 and now with more than 225 staff, John Brown Media is headquartered in London and has offices in South Africa, Hong Kong and Dubai.
Following the acquisition, John Brown Media will retain its brand identity. Chief executive Andrew Hirsch will report to Tracy De Groose, chief executive of Dentsu Aegis Network UK & Ireland.
Hirsch will also work closely with Ben Wood, global president of performance specialist iProspect to integrate John Brown content capabilities into iProspect’s programmatic and real-time advertising solutions.
De Groose said: "As one of the world’s largest content agencies, the acquisition of John Brown Media will add another dimension to our content offering across the group.
"More clients are asking for content-led strategies and we’ll now be able to develop more innovative, content-rich solutions for them, and also accelerate our growth in this space. We are excited to welcome the John Brown Media team to our business."
Wood noted the "major shifts" taking place in digital communications, moving from the world of bought media, to one where earned and owned media often play the lead strategic role".
He said: "Successful search campaigns are increasingly reliant on the ability to drive audiences towards quality, editorialised content. By aligning publishing into our search and performance offering, iProspect creates real differentiation."
Hirsch said: "We have been producing brilliant content for some of the world’s most prestigious brands over the last 20 years.
"Now, as part of Dentsu Aegis Network, we will have the ability to work alongside some of the world’s leading strategists to deliver smarter content and drive even better results for our clients."