Both companies are part of the DAS Group, a division of Omnicom.
FleishmanHillard said the move brought together professionals from news media, public relations and marketing to meet the growing need by brands for engaging content.
Specialist chief executive Niki Webb said: "Brands are driving huge spend shifts out of paid media into owned channels. We are perfectly placed to help them with this move; our expert newsroom team will be enhanced by the skills and global scale of FleishmanHillard, especially in social analytics and listening."
Specialist’s clients include Peugeot, HSBC, Henkel and Specsavers and FleishmanHillard said clients could create branded content hubs that delivered "high-value content with editorial integrity".
Specialist began 45 years ago as a contract publisher for brands but has since evolved into a full-service content agency.
John Saunders, FleishmanHillard’s president of Europe, Middle East and Africa, said: "We are already working together on digital projects, and this move makes a great deal of sense for FleishmanHillard as it continues to grow. We are witnessing huge demand for content production on a larger scale. Niki and the Specialist team will further build on our considerable content strategy and production skills."
Dave Senay, FleishmanHillard president and chief executive, said the new digital landscape was driving a "new reality" of brands as publishers and that demand for digital content had outgrown the resources of most brands to deliver it.