The operation is intended to offer clients a seamless creative and media strategy by providing integrated media and creative communications solutions.
The team will work together on a project basis, drawing its creative resource from Karmarama, with RWC Plus providing the media planning.
The venture, which will be overseen by Ben Bilboul, a joint managing partner at Karmarama, has been operating on an unofficial basis, but it will now be made formally available to existing clients. Separately, the outfit will search for its own business.
Naresh Ramchandani, the co-founder of Karmarama, said: "We have found that working more closely with RWC Plus has opened our minds and improved our work on clients such as Ikea, Opodo and Amnesty International.
"Karmarama has always offered fresh thinking - working more closely with RWC Plus will give us the expertise to do so for media solutions."
MediaCom's RWC Plus team was launched at the start of 2004 to specialise in integrated communications projects both inside and outside the MediaCom client roster.
The RWC Plus director, Matthew Mee, said: "There are a lot of people theorising about getting media and creative agencies working together again, but not many people are really doing it.
"This relationship offers clients creativity, integration and a massive research resource all in one place."
Karmarama, which bills itself as a "communications design agency", was set up in 2000 by Dave Buonaguidi and Ramchandani.