The agency won the business after a pitch through the AAR. The final shortlist was Kitcatt Nohr, Clark McKay & Walpole and Heresy.
Leonardo previously handled the business and last year created a successful integrated campaign. The agency did not repitch for the business.
Kitcatt Nohr will create brand advertising for VSO as well as the recruitment advertising it requires to attract around 1,000 professionals to fill volunteer positions.
Glyn Williams, the head of marketing at VSO, said: "We were looking for an agency that chimes with our values and shows passion for our cause, as well as combining strategic and creative excellence. Kitcatt Nohr stood out because its thinking went beyond advertising and awareness building, to a recognition of how we could use communications to develop relationships with all our audiences in an integrated way."
Marc Nohr, the managing partner of Kitcatt Nohr, said: "Our task involves raising the status of international volunteering - so that it's seen as serious professional development rather than time out."
Recent activity by VSO included press advertising featuring real volunteers who had returned from two-year voluntary projects with new skills and experiences. Leonardo also created activity to support an ITV1 documentary that followed the fortunes of volunteers called VSO: Making a Difference.