The agency, launched in January by the former Manning Gottlieb OMD directors Andrew Stephens and Ben Hayes, will work with Knickerbox on consumer research, brand positioning, communications planning and creative ideas.
The company has been appointed on a retained basis, following Knickerbox's decision to aim its lingerie at a broader range of women.
Goodstuff already works for Ann Summers, which owns Knickerbox. Knickerbox has 25 dedicated stores and 43 concessions within Ann Summers outlets.
Knickerbox does not retain a creative advertising agency and will work on a project basis when required.
Stephens, a partner at Goodstuff, said: "We have completed a Knickerbox brand positioning project, which indicates a real opportunity for the brand. We will be developing this positioning with our first work, which is expected in early 2005."
Gordon Lee, the head of marketing at Knickerbox, said: "Knickerbox doesn't have a history of advertising but, with a new product range and exciting plans for the future, finding a communications partner was important.
"Goodstuff was chosen for its understanding of our brand and how it translated this into some really interesting creative communication concepts."