Farrell, who fulfils the same role for Edinburgh City Council, will oversee an attempt to improve the urban design in towns such as Rochester and Chatham.
Bosses at Medway Council and quango Design Excellence in North Kent (DENK) said they were keen to work with Farrell because of his public interest in the Thames Gateway area, which takes in north Kent.
He has long advocated the creation of a Thames Gateway park and the restriction of new homes on greenfield land.
Speaking last night at a special RIBA reception, Farrell said he was delighted to be taking on the role.
'DENK, the Kent Architecture Centre and Medway Renaissance are creating an exemplary environment in which to build good design. I am thrilled to be involved.
'I have a particular interest in the future of the Thames Gateway and my role as Design Champion for Medway will be complementary.
'The five Medway towns and their relationship to their natural environment setting will be central to the Design Champion role.
'I am delighted to have the opportunity to help create a new community, which will be integrated in a sustainable way into an existing locality at the heart of the Thames Gateway,' he added.
The Medway towns are Rochester, Chatham, Gillingham, Strood and Rainham.