Fearnhurst will help the chain with its plans to take a fresh direction that exploits the brand's French roots and heritage.
The agency, whose MD is Samantha Fearn, will also publicise plans by Kookai to refurbish all of its stores with boutique-inspired interiors.
The fashion chain called in administrators Deloitte last year after a slump in sales. It is now owned by a joint venture company comprising previous owner Kookai SA and Amery Capital, an investment firm led by former Marks & Spencer executive Maurice Helfgott.
Kookai sold off a number of stores last year after it went in to administration.