Historically, the agency had split its outdoor business between Portland and Outdoor Focus - a hangover from the arrangements that were in place before the merger of The Media Edge and CIA at the start of this year.
The WPP-owned Portland was The Media Edge's agency of record while CIA used Outdoor Focus - a joint venture between the Alban Group and CIA.
James Whitmore, the managing director of Mediaedge:cia, said: "This move makes absolute sense and will help us provide an exceptional service to our clients. Mediaedge:cia is one of the UK's largest buyers of outdoor space and Portland, as the UK's second-largest outdoor specialist, is well positioned to leverage the best deals and packages for us."
Mediaedge:cia's business moves into Portland from the beginning of next year.
Following the realignment of the business, Portland has appointed two senior members of staff from Outdoor Focus. Julia Wilkinson has joined the board as a board account director and Sophia Matthews has been employed as a senior account manager.
Alex Thompson, the managing director of Portland, added: "Coming on the back of tremendous past two years' growth of 35 per cent at Portland, I am confident that this latest move will help propel us towards the number one positioning in the UK specialist market. More importantly, it will further improve our offering to our portfolio of existing clients."
The realignment narrows the gap between Portland and the Aegis-owned Posterscope and follows last month's decision by both OMD and Walker Media to move their respective business into Posterscope.