Naked will work closely with RBLM on brand and communications strategy.
The two agencies have previously worked together on some of Naked's Nordic clients.
Ikea's UK media planning and buying remains with MediaCom, but the retailer is currently conducting a global review of its media business.
Naked has been appointed on a retained, rather than a project, basis.
Its first task will be to work with RBLM on a campaign that is due to launch in September.
Ikea split from Karmarama after five years, during which time the agency created work including the "Live UnLtd" ads and the recent "elite designers against Ikea" activity.
The agency appointments were made by Peter Hogsted, Ikea's managing director, and Peter Jelkeby, its UK marketing director.