The airline, which is majority owned by the French Polynesian government, had about 15 "ambassadors" in sports and the arts and under the deal with Williams would provide travel support for Williams and her coach.
Williams would surf in places the airline flies to including the United States, France and Spain.
Support for surfing was a "good fit" for the airline, with travellers targeted including baby boomers interested in the sport.
He said the older, discerning traveller would not necessarily be up to surfing the big waves but still were attracted to the sport and there were alternative spots where they could surf.
Air Tahiti Nui operates five times a week from Auckland to Tahiti, twice with Air New Zealand aircraft.