The deal, estimated to be worth £500,000, runs until the end of 2009, with a view to extending the deal next year.
The agreement means Plakativ Media sites will now be sold by BlowUp alongside the BlowUp portfolio, offering national scale to agencies and advertisers planning campaigns.
There will not be any job losses at Plakativ because of the deal.
Dominic Gesua, managing director of BlowUp Media, said BlowUp would benefit from some "great locations that complement our portfolio" and Plakativ would benefit from BlowUp's "experienced sales team".
Harvey Glenn, managing director of Plakativ, said the arrangement would "play to the strength of both companies".