Marshall only moved to Dialogue DLKW in late August to become its head of digital and managing partner. Before that, he was the marketing director and business director at Agency Republic, heading accounts including Unilever.
It is thought that Marshall wants to return to an agency that is 100 per cent focused on digital marketing, as opposed to Dialogue DLKW, which is closely affiliated with its sister ad agency and specialises in direct marketing.
Martin Brooks, the chief executive of Agency Republic, said: "We've missed him and are absolutely excited to have him back, both for the agency's development and for his career development."
Marshall had been with the Omnicom-owned Agency Republic since it was founded in 2001. He will rejoin as the business director in January.
Marshall had been hired by Dialogue DLKW to head its digital operations on some of the agency's key clients, including Halifax, COI and Tena. He reported directly to the chief executive, Paul Biggins.